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Roger, BMW Master Tech
Category: BMW
Satisfied Customers: 5890
Experience:  10 yrs. experience as BMW Master Technician, shop foreman and dealer.
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1998 740iL: cats&O2 sensors..fuel pump relay

Customer Question

My 1998 740iL (120 kmiles) often has long cranking and sometimes stalls. Otherwise perfect. I have put in new fuel filter, fuel pump&relay, new spark plugs, new cats&O2 sensors last year, cleaned throttle and air flow meter, new fuel pump relay. Please make suggestions.
Submitted: 8 years ago.
Category: BMW
Expert:  Roger replied 8 years ago.

Hello Crispin,


Do you see smoke from the tailpipe when this happens or when it finally catches?

Customer: replied 8 years ago.
No, none at all. The car runs beautifully otherwise. A year ago I had to replace the intake manifold cover at the rear of the engine because I had tons of smoke, but this was way before this recent problem arose. I have posted additional details.
Expert:  Roger replied 8 years ago.

Yes. That is the common problem I was thinking about. Another common cause of a long crank is the camshaft sensors and/or the crankshaft sensor. If the camshaft sensors do not see the correct signal it will crank longer and then use the crank sensor as a reference for starting. Do you have a scanner that can read the adaptation values -multiplicative and addaptive of the mass air flow sensor?

Customer: replied 8 years ago.
I just used my fault code reader to reveal in 0b: 6c,cc,8b, a7. The 8b=electric thermostat final stage I've seen before. Regarding the a7=electric fuel pump relay, I did think that I'd found it when I noticed corrosion on the relay, cleaned the pins and replaced with no problems for a week. ( I do have a water leak in the back somewhere so that there is corrosion in the rear lights passenger side and perhaps in the fuel pump harness above the battery). Then I took it out to buy a new one and even with the new one in and me spraying sockets clean the problem resurfaced. I also replaced the two fuel pump fuses with new ones. It is hot in California. This morning it started immediately, no problems whatsoever, but I know it will happen again!
Expert:  Roger replied 8 years ago.

Hmm. You should try to find the location of the water leak and repair it. Water can damage a wiring harness and can very expensive to repair the damage. I recommend using a friend with a light and spraying down the area with a hose (friend in trunk with flashlight....I guess it should be a good friend) and look for how the water is entering. Is it the trunk seal or throught the taillamp itslef. Once determined you should do this first. You should then repair as much of the damaged harness as possible. Including replacing the pins in the relay holder(harness side) and cut back as much corrosion as possible. It is always best to start with the obvious. Perhaps once the water intrusion is corrected it will stop this problem.


As for the electric t-stat. I have seen some strange things with this problem as well. You should have the t-stat replaced and check the electrical plug to the t-stat as it often times melts from the faulted t-stat.....I have seen a melted t-stat connector cause no start situations and intermittent no start situations and it is a direct input to the engine controller so if it pops the fuse in the fuse carrier in the E-box the engine might not start until this is traced down. One final thing I have seen....and it was only once...the electric t-stat melted at the connector and coolant was being sent up through the wiring harness and all the way to the DME (engine control module) and caused the module to short out from coolant in the DME. It took me a bit to trace down this problem. That is what I suggest at this point. I hope this leads to finding the source of your intermittent crank problem.

Roger and 3 other BMW Specialists are ready to help you
Customer: replied 8 years ago.
Thank you for encouraging me to look again at the harness. I used a paint brush to squeeze in some CLR (probably phosphoric acid) that cleans away corrosion into the pin sockets and fuse sockets in the harness, rinsed thoroughly with water, and dried carefully. And, yes, only the word "voila!" will do- the long cranking and occasional stall have both gone away. Thanks!
Customer: replied 8 years ago.
OK, BmwMasterTech, this car just might kill us both off. Over the weekend, I put in new spark plugs (4-electrode Bosch) with an upcoming SMOG test, put in a new air temp sensor, a new electric thermostat and a new mass air flow sensor. First, you were right that the electrical connector to the thermostat was melted horribly so I soldered on a new connector and installed the new thermostat. So far, so good. Also, no white smoke In restarting, I noticed a slight misfiring and went through a series of runs over a few miles for the computer to "learn" how to drive, returned to the house and then on came the check engine light. I now have errors of engine misfire and cat-damaging misfires in cylinders 5, 6, and 8 (all on one side). The car also runs a little rough (but barely noticeable on the freeway) with a slight judder and very slight bluish smoke on that side of the exhaust, so it really does seem to be misfiring. Also, it has the long-cranking on most starts, hot or cold, but the cranking is a high-rep-rate (witha sound of 5-10 fires per second) as if the engine is almost starting (sounds like a machine gun). I checked the spark plugs- they are correct and correctly installed, as are the coils (all only a year old). I have always prided myself in being able to fix any car problem (6 cars in the family) but now I am seriously thinking of crying. I did notice that only on that side of the engine, the two rear spark plugs had a half-teaspoonful of oil on top of them.... but all (old) spark plugs looked very good. Any suggestions? I do not want to damage my one-year-old new cats and O2 sensors.
Expert:  Roger replied 8 years ago.

Hello again Crispin!


Wow! Sounds like things have gone bad. I will try to help you as best as I can.


The thing that stands out by your description is the oil in the spark plug holes. This is caused by a brittle valve cover gasket that then seeps into the spark plug holes over time. This causes the boots on the coils to deteriorate and sometimes fail. I would re-remove them and see if they are oily. I would them clean them very well with brake clean and spray some inside the boot hole and them blow out with compressed air to get any oil that might have gotten into the coil could be a contributing factor for the misfiring.


The next things is that you replaced the spark plugs. I imagine the oil that was trapped from the spark plug then goes into the cylinder. While this is not overly bad(it should burn off) it will produce smoke for longer than one might think(bluish smoke) as it gets into the honey comb of the cat and needs to burn off. Once in you can't get it out except by time and yes this is a damaging factor that could cause a cat to fail.(sorry I like to be completely honest and not sugar coat problems.) However, most of the time no failure results unless it is a lot of oil like an oil hydrolock problem. I would remove the plugs and clean them as well and then reinstall. Make sure everything is dry and clean before reinstalling both the plugs and the coils.


Also, did you use a BMW mass air flow sensor or and aftermarket mass air flow sensor? I have seen many problems with non-BMW mass air flow sensors. You could also try to install the original mass air flow sensor and see what happens. I hope this helps you Crispin!

Customer: replied 8 years ago.
Good call on oil in the boots! There was oil trapped, I cleaned all 4 thoroughly and the blue smoke and cat-damaging errors went away. However, a new problem has arisen (idle surge) and the fast long cranking (every other time) remains. The only error code I had was for the throttle control valve stuck mechanically so I cleaned it inhternally with brake cleaner and the electrical contacts, and that went away. SO, right now it drives beautifully on the freeway, but has the idle/long cranking. When stationary in the driveway at temperature, it seems to be steady in drive, but in "park" it is generally OK, but when I turn off the AC it goes from 600 rpm up to 1700 rpm and wanders down slowly or sticks high... Also there is often a high pitch humming sound that I hope is not the water pump. I did buy a Bosch mass air flow valve and have tried putting the old one back with the same results. Also, I rechecked an recleaned all emission related electrical contacts. So far, you're fixing the problems as I develop new ones, but can we leapfrog to perfection?
Expert:  Roger replied 8 years ago.

I am glad I was right about oil on the boots. (been there :) )As for leapfrogging to

perfection....I wish problems like this were that easy my friend. I have been thinking

about your symptoms. A long crank can be caused by many things. A failing crank sensor, failing cam sensors, poor fuel pressure or volume.


I like to look at the big picture with problems like this. Because you stated you have

the problem switching form AC to no AC it sounds like it could be a failing Idle Air

Control valve where it is getting mechanically stuck and can not adjust itself fast


Also, you have done a lot of work relating to air and fuel. The dme has a stored value

called adaptaion values. They should be cleared to start the DME at zero so it can now

relearn how to control the air fuel mixture. I am not sure if that is something that can

be done with a normal scanner.

Roger and 3 other BMW Specialists are ready to help you
Customer: replied 8 years ago.
O.K., so things are improving now, but perfection lies just ahead (as always). It seems that several things are going at once, and confusing the picture. The smoke just suddenly got worse as did the engine surging with or without the AC. SO, (i) I put in a new idle control valve - and the idle is rock steady with or without other electrical on or off; and (ii) In frustration I changed the oil-vapor separator plate/valve a the back of the engine (that as a I said I had to do only 10 months ago!!) and WOW- it got rid of the smoke! The car drives like new around town/freeway with awesome power and idles beautifully and solidly EXCEPT (i) I have a new fault code A8=Idle speed actuator (open) that returns even when I reclean connector on NEW idle control valve; (ii) it STILL struggles with long rapid cranking, sounding like it is a mix of going itself and cranking and will then suddenly start on 3rd or 4th attempt. Now with new plugs, AMF, Idle contro, air temp sensor, new fuel filter and pump, new O2 and cats a year ago, I'm wondering if the fuel pressure regulator is next. What thinkest thou?
Expert:  Roger replied 8 years ago.
<p>Hello Crispin!</p><p> </p><p>You must be a "glass half full" and not "half emply" type of person because through all your problems you always have a positive and amusing way of presenting the problem.</p><p> </p><p>It is great news that progress is being made! For FC A8=idle actuator open I will need some time to refer to some wiring daigrams. Since the idles has improved it would seem logical the your DME is controlling the IAC valve successfully. This would be a direct input to the DME so I will try to find you pin numbers at the DME and IAC connector so you can check the wiring.</p><p> </p><p>The long cranking..........On your vehicle you should check to see if you have a shrader valve on the fuel rail(this is the metal rail that feeds the injectors). If you do have this fitting it should have a metal cap on it similar to a tire valve stem cap. The best way to check for a fuel problem without just guessing would be to check the cranking pressure, running pressure, and bleed down rate...oh and also the prime pulse.</p><p> </p><p>You will have to purchase a fuel pressure gauge, but it should not be too does not have to be high end but would need the shrader valve screw in fitting. Before a make a long winded explanation of what to check...can you see if you have the fitting required for this test and let me know....and I will continue on what to check for.</p>
Expert:  Roger replied 8 years ago.
<p>Hello Crispin!</p><p> </p><p>What I wanted to check for the possible fuel problem was fuel pressure. With a fuel pressure gauge attached you can see if the pressure spikes when you turn the key on-not started-this would be the prime pulse. How fast does the prime pulse drain off. It should hold for at least three minutes. Also what is the cranking pressure, if the pressure is less than 3 Bar or aprox 25-30 psi then this would indicate a fuel pressure problem. Also, after running the engine you can watch the fuel pressure and see how fast it drops off to 0. The pressure should hold some pressure 10-20 psi for aprox 5 minutes and if it steadily drops that means the check valve in the pump is bad.</p><p> </p><p>I just re-read the original post. I see you have already done the fuel pump and filter. It is possible it is the regulator and if fuel pressure is not attained during cranking that should be what replace. Although it could still be a faulty pump but the best way to verify this would be the fuel pressure testing.</p>
Customer: replied 8 years ago.

To summarize, your suggestions to date have been great, but the long cranking (9/10 times) remains. The FC A8 error I buzzed out to the DME and discovered a near-break in the wire near the idle control valve, fixed it and THAT problem went away. Also, I bought a good fuel gauge, put it in line with the rail, and saw correct pressure spike, excellent cranking pressure and 40 psi on idle (high frequency flutter with fuel injectors), with the pressure holding even overnight when engine off. Interestingly, the fuel pressure is always solid and in specs even during a long-cranking failure- no hint of any fuel issues. Note that I have a new fuel pump, filter, mass sensor, idle control valve, spark plugs a month ago, coils a year ago, O2 and cats a year or so ago. I now have NO error codes, no check engine light, and the car runs superbly EXCEPT for the starting and especially when at temp. Again, if we focus on electrical I would note that often during the long crank the car is trying to run fast, i.e. about 50% crank and 50% running. Flooring the pedal before, during or after makes no difference. I read on line about ignition switch problems, but am loathe to go change it without better data. What do you think? The long cranking is so bad that I have to re-charge the battery at night and occasionally will not get it in the 12 or so cranks before battery depletion. Well, now that the cabernet sauvignon is taking effect, let me share a spooky observation with you. Nearly every time that I think that I have found the problem (e.g. fuel relay, bad fuel hose, bad connectors) and make a change, the cranking is fine for about a week during which I feel like I have the upper hand, and then, about a week later, it reverts back to its old ways.... As the scientist who insisted that the thermos flask was the greatest invention because it worked hot OR cold, I would repeat his words: "But how does it know?"

Expert:  Roger replied 8 years ago.

Hello Crispin,


Since fuel pressure is might be time for an exorcism (at least it would run good for about a week).


What your found online is correct. This vehicle does have a lot of problems with ignition switch failures....Usually the complaint is that the radio cuts out at times, or the dash lights flicer, or various electrical problems....there is an older tech that would start the vehicle and lower the visor on the driver side to turn on the upper lamp. When he did this the electrical problems would happen. This would indicate a bad ignition is something to try. I do not know if this could be an ignition switch problem. Even though it is a common failure, it does not fit the symptom quite right. I will try to review some wiring diagrams to see if it makes sense.


When the long cranking it after a rain or a wet day? Can you explain the 50% crank and 50% running in more detail? Does it spin fast with no compression sound? Stumble while cranking like it is misfiring? Do you have another key?---If the EWS is not getting the proper signal from the key it will crank and not start, also it could be an intermittent EWS problem in the module. EWS=electric car immobilization and prevents the dme from releasing injector fuel and spark. This is only a guess. I am out of to Beamer land...but I will do some more thinking on this...


By the way...Great find with that wire at the throttle!


Customer: replied 8 years ago.


All these car troubles are occurring in dry southern california weather and not rain dependent. I have NO troubles or events with any other electrical systems. Regarding EWS, the Bentley manual I bought tells me that for my particular model and date, the engine will not even crank without the key signal (I tried the second key anyway with no change). Note the car runs beautifully now and never stalls or shows any issues - only the start! 70% of the time the long cranking is normal sounding with compression, but 30% of the time it cranks really fast, with a weak compression feel, trying to run on gas (kind of hyperactively) with a slight ring sound of the flywheel. In a sense, it is tripping on its own timing it seems, being just ahead or behind of the sweet spot. Also, it never starts if I let it long crank- rather it starts immediately (WHEN it starts) when I turn the key. So I turn the key and kill it if it doesn't start right away. Typically it now takes 9 or so efforts to get started. Is it a crime to kill a car?

Expert:  Roger replied 8 years ago.



I think it would only be a crime if you use a shotgun on broad daylight.


I checked the wiring diagram, you are correct, the ews is also the starter relay so if it was failing the vehicle would not crank. Reading your description makes me think the 30% with a ring of the flywheel is like the starter is not engaging the flywheel correctly. This could be a failing starter or a worn out flywheel (from starter engagement) The slight ring sound from the flywheel makes me think this. You could remove the starter and take a look at the gear to see if it looks worn and also check the flywheel.


Crispin, has the crank sensor been replaced? The DME is looking for the crank sensor signal to know when to fire the injectors. If you are getting a bad signal it could take a few cranks before the DME figures out the position. This does not always set a check engine light or a fault. It is just a sensor so it is not too expensive. I hate to have you throw parts at it but it is a possibility.


If I had your vehicle at the dealer, we could check the signal or scope out the pattern while cranking to see if the signal is getting to the DME. The other item I would like to do is to reprogram your DME. This might be something worth trying as well. At the dealer we can see a lot of inputs using our equiptment. The good thing is that it is reproducable...not for you though...but it helps us to see the problem so we can check the inputs to see if something is not right. I hate to say it but it might be worth a run with the dealer to take a look at what is going on when the problem occurs.


I hope I am helping....let me know you thoughts and what you feel your next step should be. I have been enjoying trying to help you. If you would like to drive out to CT I will scan it for you :) I hope I can get you past this last hurdle (without the use of firearms!)






Customer: replied 8 years ago.

Dear MasterTech,

I did change the crank sensor, but no change to the difficult start, but now have some important new info for us to enjoy (!) Knowing now that the fuel side was fine, I removed coil #1 and attached it to a new spark plug that I carefully grounded with Cu wire to the engine, enclosed the end in a cone of black paper for easier viewing and had someone do the crank thing. When it starts right away (now 1 in 5 times and getting less often it seems), there is a strong spark, but when it does its long-crank-no-start thing there is NO SPARK every time (which explains why it doesn't start!) Now, FYI, all the coils are new upgrades from Bimmerparts about a year ago, so I do believe that the lack of spark reflects the lack of low voltage input... Also, remember, I still HAVE NO ERROR CODES or engine light, and the car runs beautifully once started. However, today for the first time, it died on the freeway, but I successfully restarted it in neutral by tapping the break etc. SO, knowing it cannot be the security system, I'm thinking that maybe it is the ignition switch, but could that cause death during a drive? What sayest thou, O master of the BMW?

Expert:  Roger replied 8 years ago.



Good to hear from you. We WILL fix this car!


I took out my magic eightball to try and help me fix your 7 series. After a good shake it asked if you have replaced the DME main relay? Smart little eightball.


As for the ignition switch....all I could say is maybe. I have seen a bad ignition switch keep a vehicle from shutting off, so why not causing a shut off problem. It is plausible that if the incorrect signal is being sent it would be like turning off the key. I think it would be a good idea to do, since it is a common problem. If you have been getting any strange electrical problems like intermittent radio shutting down this would be a good guess.


By the way, nice find with the spark!

Customer: replied 8 years ago.
Dear BmwMasterTech,
I replaced and recleaned the DME main relay with no change, and moved on to the ignition, removing the shroud and electrical connector to reveal little holes into the ignition contacts. A blast of electronics cleaner (probably MEK) into the ignition switch did the trick! The car has started now 30 times in a row, going strong, with no other errors. My God, I do believe that we have found our solution! Thanks so much for your excellent advice and help with the myriad of problems that this old lovable car has put me through. I'm delighted to be (safely) speeding confidently along the freeway with that all that quiet power that the 7-series offers without the fear of sudden engine stall. Until the next (problem) time, Thank you!
Expert:  Roger replied 8 years ago.



What a trip it has been. It made me feel good that I was able to help you figure this out, though I must admit you have a pretty good grasp on things yourself! I hope she treats you well from here on out. I get a laugh over the progression from wanting to shoot the car to "this old loveable car". It has been enjoyable for me assisting you. Maybe I will get to help you again in the future. Take care and I wish you well!

Customer: replied 7 years ago.


Although I hate these long goodbyes, I wanted you to know that I had yesterday added a $30 bonus for all your hard work (one hundredth the cost of a BMW dealer visit) so that you can enjoy a decent Cabernet Sauvignon (but I'm guessing you're a single malt Scotch drinker). I did also have one last question please since I am trying to pass a smog test. After you and I produced a fully-working car, I reconnected that battery, but the codes that normally accompany this action =6C (battery disconnect) and CC (EWS rolling codes) persist (no other issues) for the last 5 days and I am worried that they will cause an automatic smog test failure. Should I (i) keep driving since they will one day soon disappear or (ii) use my code tool now to reset all codes, then redrive for a few days for the OBD to relearn?? Will patience be a virtue or a curse in this case?

Expert:  Roger replied 7 years ago.

Hey Crispin!


Thank you Crispin for you bonus, it is much apreciated. I was in a hurry to read your recent post to see what new problem has popped up now that everything else has been resolved (knock on wood.) I think it would be safe to say that all you went through would drive anyone to drink (a single malt Scotch does sound good!).

I have hear California is one of the strickest states for the smog test. I think the battery resets might be a problem. Your best bet would be to ask the testers about that part. If it does not matter you should be set to proceed. If you use the scan tool, you have to wait for all of the test in the vehicle to run. This could take quite a few drive cycles. If you have time this might be the best bet. If you do clear the memory, you will want to drive the vehicle as much as possible until you go for the smog test. Possibly two or three tanks of fuel worth. Many tests have to run including, catylitic converter efficiancy, purge valve test, secondary air test, LDP(leak diagnosis test)....I actually had a z3 that took 3 months due to aging 02 sensors for it to pass.

You have done quite a bit of work so maybe a clean slate and some time driving would be a good idea, but before you take that step a quick call to the local smog tester might shed some light on the battery resets and ews. I think the only thing they would be concerned about would be the battery resets as the ews does not effect smog. Up in CT, we get a two shot deal. If we fail then we get to go back for free for a second test later within a certain time. If it is the same for you, it might be worth it to try and see if it passes. At least you would know exactly what it failed for and can clear and drive for next time.

It has been a pleasure helping you. Is it wrong to hope you car breaks so I can read your posts back? (Just kidding about the car breaking thing! Your 7 certainly put us to the test!) If you need anything else, I will give it my best to help. Bye for now Crispin.