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1998 GMC: Engine developed a miss at idle a few weeks back

I have a 1998 GMC...
I have a 1998 GMC Savana with a 5.7 Vortec. Engine developed a miss at idle a few weeks back...now stumbles on acceleration as well. Replaced cap, rotor, wires, plug, fuel filter with no improvement. No SES illuminated.

I unplugged the cam position sensor (hall effect switch) in the distributor and all problems went away other than the SES comes on. Ran like new. Replaced sensor and plugged it back in. Still has previous miss problems. Unplug it and problems go away.

Crank Pos Sensor, MAF and 2 front O2 sensors were replaced last fall as well.

I have no diagnostic tools so at a bit of a handicap.
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Answered in 3 minutes by:
6/10/2011
kalamykid
kalamykid, Shop Owner/ Tech
Category: GM
Satisfied Customers: 4,560
Experience: I HAVE OWNED MY OWN SHOP FOR OVER THREE YEARS. I HAVE THE TOOLS AND SOFTWARE TO ANSWERE ALMOST ANY Q
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kalamykid :

When you unplug the cam sensor you send the pcm into a base line setting for fuel delivery. If unplugging the cam sensor makes it run correctly, then The first thing I would check would be the fuel pressure. If the fuel pressure is below 60 psi, then you will have one of three problems, the fuel filter is stopping up, the fuel pump is not pumping hard enough and needs to be replaed, or the fuel pressure regulator is not holding pressure and needs to be replaced.

Customer:

i will have to see if the local parts store has a fuel pressure guage

kalamykid :

ok. let me know what the pressures are.

Customer:

ok...first tiem I have used this...I can let you know tomorrow?

kalamykid :

yers that will be fine. do you know how to get back to tehis string?

Customer:

no

kalamykid :

Look in you r email and there is an email from just answer. In that email is a link " get your answer here", just click on the link and it will bring you right back here. Then post a mesasage to this string and i will be right back with you asap.

Customer:

perfect...thx. tty later

kalamykid :

chat with you alter.

Customer:

well...everyone should own a fuel pressre guage, right? it tests 56 lbs at idle

Customer:

at key on/key off when the pump runs it jumps to 64 lbs

Customer:

when you shut off after running it holds pressure at 56lbs

kalamykid :

Thoise are good pressures.

kalamykid :

Now i have to find another direction to go in.

kalamykid :

Can you unplug the ect " engine coolant temp sensor?

kalamykid :

The ect will be up next to the thermostat housing, and will have a yellow and a black wire.

kalamykid :

See if the engine will start and run better with the ect unplugged please.

Customer:

ok...brb

kalamykid :

thank you

Customer:

may have to wait for daylight....plus its raining!

Customer:

I will give it a try in teh morning

kalamykid :

That sounds good. i will be back online around 10:30 am eastern time.

Customer:

ok...glad you are leaning toward not being fuel pumo or fuel pressure reg...bot llok nasty to change!

kalamykid :

Yes they are a pain to change, and the fuel ppump is not so cheap.

Customer:

tty tomorrow

kalamykid :

Chat with you then.

Customer:

Disconnected the ect. Started engine which idled rough and accellerated poorly. Engine was "shuddering". Drove for 10 miles and got temp gauge to normal operating temp. Performance and engine miss did not ever get any better.

Customer:

Unplugged Cam Pos Sensor and ran great.

Customer:

Not sure how the ect works other than its resistance changes as the temp changes. So should it run bad with it unplugged?

Customer:

BTW...having the ect unplugged made no diff. the vehicle behaved the same as when it was plugged in.

kalamykid :

Good morning.

kalamykid :

Let me look up a few things.. The ect works as you are thinking, but it too regulates fuel mixture based on temp. Did the check engine light come on with the ect unplugged?

Customer:

it's been on due to the unplugging of the CPS

kalamykid :

Where any new codes set?

Customer:

sorry...I dont have a code analyzer...shade tree mechanic

Customer:

I miss the 70's

kalamykid :

Be right back.

kalamykid :

Here is what I ahev found out about your cam sensor. The cmp should not cause a miss or any driveability problems. It is only used to detect a missfire in the engine. YA right, we both know that that is not true as you can unplug it and the engine will run right.

Customer:

yup

kalamykid :

Have you looked into the back side of the mass air flow meter to see if there is any dirt on the sensors inside of it?

Customer:

they looked clean to the naked eye

Customer:

you could see the tiny little wires crossing the other wires

kalamykid :

and there was no dust or dirt on the tine wires?

Customer:

did not look like it

kalamykid :

HOw many miles do you have on the engine?

Customer:

last fall when the MAF failed, if you unplugged it , it ran better...if you unplug it today it makes no diff

Customer:

110k

kalamykid :

If you have thr cam sensor unplugged will unplugging the mass air change the way the engine runns?

Customer:

not tried that...brb

kalamykid :

ok

Customer:

ok...ect plugged back in...MAF unplugged...Cam sens uplugged...runs good

Customer:

plug cam sens back in with MAF unplugged and runs bad. like flipping an ON/OFF switch

kalamykid :

I hat eto say it but I believe the pcm is where your problem is. there are two ways to go about correcting the pcm, you can try to have it reprogramed, or get a new or used pcm. If you replace the pcm the new or used pcm has to be reprogramed before it will work in your van.

kalamykid :

There is an outside chance that the crank sensor is bad,

kalamykid :

But I would not place money on the crank sensor. " not much of a gambler".

Customer:

the crank sensor went out last fall and was replaced at a dealer in Liberal KS. I now am missing a left arm and leg by the way...but I suppose it could be faulty

Customer:

we were on a vacation and it shut down on the side of the road

Customer:

he "put two guys on it" to get us on our way. funny it took the same amount of time as one.

kalamykid :

Is it posible that the distributorwas loose and the distributor has turned?

kalamykid :

I hate the " look out of towners, lets get all we can" shops

Customer:

I dont think so...I did read about how the gear on the dist shaft can wear...

Customer:

it's been a gradual thing over the last few weeks ...started out running rough just when u first started it to running rough all the time

kalamykid :

I have heard people say that the gear cn wear, but i have never had to replace a gear for the distributor, unless the distributor was sizing up.

Customer:

I first had thought bad gas but ran that tank out and fill up at a place we get gas all the time at wioth no issues on our other cars

kalamykid :

if the gear was the problem, then the miss woudl not change with unpluggingthe cam sensor.

Customer:

replaced fuel filter...old one had a little resistence but was a yr old

Customer:

good point

kalamykid :

I hate to say it but it sounds like the pcm is breaking up the signals it is getting.

Customer:

MAP sensor? have not changed that yet

kalamykid :

A map sensor can cause the engine to miss, but I have never unplugged the cam sensor to see if the miss will clear up, so I can not condem the map sensor, but that does not mean it is not possible

kalamykid :

A map sensor for this engine is not to bad, around $30.00. And is a hole lot less then a usd pcm and programing.

Customer:

so botXXXXX XXXXXne...we use this vehicle for short trips...not a daily driver...will it hurt anything running with the cam sensore unplugged?

kalamykid :

From what I can tell the only thing that woudl be efected would be the pcms ability to determine what cylinder is missing if you leave it unplugged.

Customer:

whats wierd is when it is unplugged no cylinder misses!

kalamykid :

I agree with you it is wierd that the miss goes away.

kalamykid :

I hate to so lets try swaping a part out, but in this case I believe that you may be onto something with the map sensor. DO you mind replacing it to see if it does clear up the miss?

Customer:

sure...at this point I will try dang near anything. looks like an ECM runs $156 at OReilly...not as bad as I thought

kalamykid :

But then you have to have the pcm programed ontop of the replacement.

Customer:

speaking of dealers...I initially took this to our local GMC dealer...he quoted me $757 to do new plugs wires cap and rotor. I told him no way.

Customer:

true...what should that normally run?

kalamykid :

At the dealer it runs around $135.00, but oreilly's may be able to program it for you. There is one store here in chat that can do it for me, put of the 7 stores near me.

Customer:

ok...I will get a MAP sensor at lunch and give it a shot...it's sort of like playing the slots in Vegas...you reach a point where you think if you just keep going you will eventually win!

kalamykid :

I hate to say it but you are right. With out a good scanner to see what is realy going on, it is a roll of the dice.

Customer:

yep

Customer:

I'll get back to you

kalamykid :

thank you.

Customer:

drum roll please....

Customer:

MAP sensor fixed it

Customer:

was a bit worried though...installed it and plugged all components back in....still ran crappy. unplugged cam sensor and drove home.

Customer:

decided to plug cam sensor back in while it was still running...no difference! still idled great. went for test drive and ran perfect. I am guessing it just took abit for the ECM to relearn.

Customer:

now at idle you cannot feel in the steering when that it is running.

Customer:

accelerates fine at all speeds.

Customer:

SES is still on but hopefully will reset after some key cycles. Is there a way to reset it? Disc battery?

Customer:

so with all tuneup parts, cam pos sensor, MAP sensor and my shiny new fuel pressure gauge I am still way less than half of what my dealer/robber wanted for a tune up!

Customer:

oh, and your $$ ;)

Customer:

meant to say above: now at idle you cannot feel in the steering WHEEL that it is running.

Customer:

btw....$75 for oreilly to have an ecm flashed

kalamykid :

I am so happy that the map sensor corrected your problem. To reset the codes in the pcm and turn off the ses, disconeect the negutive batter cable ofr 2 minutes.

Customer:

perfect!

Customer:

was i correct in my assumption that it would take a bit to "relearn"

kalamykid :

WIt not clearing the codes after you installed the map, yes that is very posible.

kalamykid :

The pcm would have had to " try" to get a good reading not knowing that the map had been changed as the pcm know that things where not right,

Customer:

great! well thanks for all your help. I just got off the phone with the local dealer and vented ! I am clicking "Accept" now!

kalamykid :

You are very welcome.

Customer:

ttyl

kalamykid
kalamykid, Shop Owner/ Tech
Category: GM
Satisfied Customers: 4,560
Experience: I HAVE OWNED MY OWN SHOP FOR OVER THREE YEARS. I HAVE THE TOOLS AND SOFTWARE TO ANSWERE ALMOST ANY Q
Verified
kalamykid and 87 other GM Specialists are ready to help you
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Customer reply replied 6 years ago

kalamykid,

 

Well...I know I closed this out but I guess it wasnt really the MAP sensor that fixed it. It appears like it was because the van had warmed up to normal operating temp and the miss went away. I am going to replace the engine coolant temp sensor and try it again. It still only runs well if the dist module is unplugged (matters not if the van is cold or warmed up...runs perfectly).

 

I do have a new litttle thing to add...there is an occasional "clunk" in the tranny or torque converter when going downt the highway like is is locking or releasing. Not sure which. Shifts through all the gears fine. Happens if I am running in OD or 3rd.

 

-Mike

The lock up converter does not engage until the trans goes into od, so the converter is out. I would check the trans mount and u joints to see if there is any play in them. Is the pcm throwing any codes now?
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Customer reply replied 6 years ago
I've been running it with the dist module unplugged or else it is not driveable. I will get the temp sensor changed and clear the codes by disconnecting the battery and then see. I think they have a scanner tool at Advance Auto to see if it is producing any codes. Will get back to you.
If you have an orielly's near you, get a borg warner ect and see if it will clear up. I had a customer the other day that went to advance and tried 4 ects, and all of them where wrong, went to orielly's and got a bg ect and it fixed his problem.
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Customer reply replied 6 years ago

good to know...I will let you know what I find...

 

I was reading that the ect tells the PCM to lock up the torque converter at certain temp as well...hopefully this will fix all my issues!

The pcm can not lock up the converter till the engine reaches full operating temp, so the ect will not tell the pcm when to lock up the converter, but does tell the pcm when it can or can not due to engine temp. I hope you understand, if not I can go deeper into this subject.
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Customer reply replied 6 years ago
Yes, I understand...what I am wondering is if the PCM is getting flaky readings from the ect after it is warmed up like "hey..I'm warm...now I'm not...now I'm hot...now I'm cold...etc" could the PCM cause the torque converter to do stuff when it shouldn't?
It is possible.
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Customer reply replied 6 years ago
Sorry I have not gotten back to you...I have been swamped at my real job and havent had a chance to change the ect sensor...hopefully this weekend. I will get back to you. Thanks.

thank you for the update.

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Customer reply replied 6 years ago
Finally got time to install the new ect. Still get the torque converter clunk. If you unplug the ect you do not get the clunk and it runs good except it is hard to start when engine is warmed up. You have to pump the accelerator to get it started then it is ok. I am wondering if it could be the ecm.

We are going to have to get down and dirty to solve this problem. YOu will need to get a digital volt meter so we can do some testing to see if we can solve this problem. You can get a cheap on from your local auto parts store for about $20.00,

You will also need a test light,

With these two tools we can solve this problem together. do not get the expensive tools as they are hard to deal with and quite frankly you just do not need the more expensive tools.

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kalamykid
kalamykid
kalamykid, Shop Owner/ Tech
Category: GM
Satisfied Customers: 4,560
4,560 Satisfied Customers
Experience: I HAVE OWNED MY OWN SHOP FOR OVER THREE YEARS. I HAVE THE TOOLS AND SOFTWARE TO ANSWERE ALMOST ANY Q

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