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Mike S.
Mike S., Chevy Mechanic
Category: Chevy
Satisfied Customers: 5178
Experience:  ASE Certified Master Technician
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Chevrolet K1500 4x4 I have a 1992 chevy 5.7L TBI. It has

Customer Question

I have a 1992 chevy 5.7L TBI. It has an intermittent miss/stumble at idle. I have replaced the entire distributor, the egr valve, egr solenoid, the IAC, the Temp sensor, plugs and plug wires 2 times, replaced the factory coil and distributor cap with an MSD higher voltage one, and the bottom TBI to manifold gasket. The compression in all in all 8 cylinders is 140 - 145 psi. Fuel pressure is 13 PSI.  I purposely have the timing set a 3 degrees advanced, (Yes I did disconnect the ESC brown, black stripe wire first)and the o2 sensor was burning nicely (light brown) so I did not replace it. The miss is more suttle now but still there. I sprayed starting fluid around the intake- no change in idle. However even with the new gasket installed when I sprayed it only a couple of times I occasionally will get an increase in idle around the TPS sensor, I do not believe it is the gasket as I followed the torque specs (12 psi) Perhaps that TPS seal is hardened?? I have never seen this but my miss persists!! Also I have no DTC's indicated.
Submitted: 3 years ago.
Category: Chevy
Expert:  Mike S. replied 3 years ago.

Perhaps it is the tps gasket. By the way psi is not a torque spec for tightening nuts, blots or screws, most likely the torque spec was 12 inch pounds.

Customer: replied 3 years ago.
Sorry about that. Was very late. I mean't ft lbs or lbs ft as some prefer. So after all I have done you think as well the TPS gasket could leak vacuum. I have checked everywhere and it seems that is a part that is not available I will check further and get back to you. Thanks for your info. Will the truck run with the TPS off the throttle body but still connected? That way I can confirm it is the rubber gasket.
Customer: replied 3 years ago.
I do not see my question in the queue. Did i mess up in trying to get a reply
Customer: replied 3 years ago.
Since I have been waiting I have did the following. Replaced the TPS with a new one and checked the gasket, reinstalled everything and still a miss/stumble exists. I removed the TPS, engine running, sprayed some starting fluid in theTPS cavity and no change in RPM. In fact I went around all the base again and all vacuum hoses no change, still an intermittent miss. Hope to here from you soon
Customer: replied 3 years ago.
I have added some additional info that may be of help to you in your diagnosis of my problem Thanks
Expert:  Mike S. replied 3 years ago.
Are you sure it's not the gasket on the IAC that is bad?
Customer: replied 3 years ago.
I replaced the IAC with new one. But I can go check it.Will be back in a minite. But I did spray around it and the RPM did not pick mup
Expert:  Mike S. replied 3 years ago.

Do a cylinder balance test and see which cylinder it is and do it a few times to see if it always comes back the same one or ones or if it moves around.

First of all you will need a 12-volt test light and about 8-12 inches of neoprene vacuum hose. The vacuum hose will conduct electricity cause it is carbon based, use an ohmmeter if you are not sure and see if the vacuum hose you have will conduct.
Now, cut off the same # XXXXX cylinders you have in small equal lengths of the vacuum hose. 2-3 inches will do fine. The small diameter kind like to carburetors, etc will work as long as they fit over the distributor cap tower connection.
Now, mark all your spark plug wires at the cap or coil pack and remove them all. Put those short pieces of vacuum hose on the distributor cap or coil pack connections and shove the other end of the vacuum hose into the spark plug wire boot until it makes a good connection.
Now connect your 12-volt test ground clip to a ground and start the vehicle. With the engine running touch the 12-volt test light to each of the vacuum hose 1 at a time and listen for the cylinder to short out and die and drop in r.p.m. They should all be about equal. If 1 or a few don’t drop or do anything than you have your dead cylinder there.
graphic

Customer: replied 3 years ago.
I do not believe I have a dead cylinder. I did a compression test and all were 140-145 PSI. All are firing, I am sure of that
Customer: replied 3 years ago.
I did a complete cylinder compression test and all are 140-145 PSI.I know all 8 cylinders are firing.
Customer: replied 3 years ago.
Can it be in the fuel diaphram/metering or pressure regulator
Expert:  Mike S. replied 3 years ago.
A cylinder balance test doesn't check only the ignition, it checks fuel and vacuum leaks. You said you "It has an intermittent miss/stumble at idle." I assume a miss is a missfire.
Customer: replied 3 years ago.
It stumbles. if yo lean on the vehicle you can fell it .on rare occasion it wll hard miss yes
Expert:  Mike S. replied 3 years ago.
Then do the cylinder balance test and let me know which cylinders are missing.
Customer: replied 3 years ago.
what is next
Customer: replied 3 years ago.
I did the cylinder balance test all were about equal. I even put in my hearing aid
Expert:  Mike S. replied 3 years ago.

When exactly is it misfiring? You are probably going to have to re-create the misfire and catch it while doing the cylinder balance test to first determine where or which cylinder or cylinders it is coming from then go from there. With all these parts you replaced though it is going to be difficult to suspect anything unless you can make it misfire and catch it in the act.

If it is misfiring at a certain rpm, without having to rev it up too high, fold up a piece of paper or matchpack and place it under the idle screw to keep it at a certain idle while you do a cylinder balance test.

Customer: replied 3 years ago.
How can I send you an email with an attac of my tbi
Customer: replied 3 years ago.
Do you not think it is fuel related and I cannot get at my idle screw as it is inside the thottle body. I want to send you an email picute of it but need address
Expert:  Mike S. replied 3 years ago.
Don't you have a toolbar on top of this box? If so to insert a picture (jpg or gif) click on the paper clip icon, right to the left of the youtube icon, it says insert/edit image if you move your mouse over it.
Customer: replied 3 years ago.
I am on a MAC OS-X Leopard version 10 4.6
Customer: replied 3 years ago.
I sent you an email hopefully
Expert:  Mike S. replied 3 years ago.

Do you see this? Do you have that paper clip icon?

graphic

Customer: replied 3 years ago.
Email on the way
Customer: replied 3 years ago.
Email with attachment sent
Expert:  Mike S. replied 3 years ago.
Email to what? I don't have an email address on here. Just upload the picture to www.mediafire.com then send me the link.
Customer: replied 3 years ago.
Expert:  Mike S. replied 3 years ago.
It has no file extension. Windows doesn't know how to open it.
Customer: replied 3 years ago.
I am on A MAC I will try to get windows running
Expert:  Mike S. replied 3 years ago.
ok
Customer: replied 3 years ago.
mediafire.com/?pww6vf3z26t3awf
Expert:  Mike S. replied 3 years ago.
Still no file extension.
Expert:  Mike S. replied 3 years ago.
See if this works, http://iconverticons.com/
Customer: replied 3 years ago.

.http://www.mediafire.com/?pww6vf3z26t3awf

Customer: replied 3 years ago.
Expert:  Mike S. replied 3 years ago.
Nope, neither of them works. The file must be converted to work on windows, but I don't know how to do that.
Customer: replied 3 years ago.
220-TBI-1
Expert:  Mike S. replied 3 years ago.
Ok, lets just forget about that file. What is it you wanted to show me? Unfortunately, a 1992 was still an OBDI and they didn't monitor too many circuits. It's getting late tonite, I'll be back on tomorrow morning but in the meantime perhaps just testing everything that is involved with the engine controls would help. For those procedures, click here http://www.mediafire.com/?knnd4m6k5wacros , just ignore any if there are not on your specific system.
Customer: replied 3 years ago.

http://www.mediafire.com/?pww6vf3z26t3awf

 

THIS OPENS FOR ME IN WINDOWS

Expert:  Mike S. replied 3 years ago.
What are you using to open it with?
Customer: replied 3 years ago.
graphicIt seems like I remember having a different type plug in it when this started. It was after I switched back to AC R43TS that this started. I am not totally positive. I had the AC Delco Truck Plug that was and upgrade from copper.graphic
Customer: replied 3 years ago.
the plugs in it are brand new r43TS and indicate a nice burn and color. Should I try opening the gap to make them a bit hotter or perhaps replace them with i hotter plug?
Expert:  Mike S. replied 3 years ago.
I would not worry about going hotter unless you had oil to burn off. Do you have any codes at all set? I know OBDI hasn't many codes.
Customer: replied 3 years ago.
no codes
Expert:  Mike S. replied 3 years ago.
I would try the starting fluid around that tps again. Is that a new seal?
Customer: replied 3 years ago.
yes and I have checked all the vacuum places and no gain in rpm, I can do it again but i need to do buy some more fliud
Expert:  Mike S. replied 3 years ago.
ok
Customer: replied 3 years ago.
Sorry I had to work. Since our last communique I have rebuilt my TBI except for the injectors. When Iapplied voltage T clicked and I can see a steady flow at idle. I did punch out the idle adjustent cover and reset the idle for 650-700 RPM in neutral and it comes to about 575or so in gear. The mss is still there but not as notiticible.It was suggested to me to swtich to an Autolite plug. These AC R43TS are burning fine. Other than that all is the same. I awit your reply. I work again tomorrrow.
Expert:  Mike S. replied 3 years ago.

If there is a misfire then doing a cylinder balance test will at least tell you which cylinder it is so then we can start testing things.

If you suspected the spark plugs after doing a balnce test you could swap the plugs and see if the misfire moves.

 

Customer: replied 3 years ago.

I have already did the cylinder balnce test as you instructed. The test revealed all abouy equal meaning the all dropped in RPM some. If you think the plugs could be the reason even though all have a clean burn I will buy some new ones maybe autolite or AC Delco Rapid Fire? Your opinion please as I am heading out the door to work in about 30 min. I do appreciate your help in trying to resovle this. One other thing do you think I should replace the O2 sensor? It has a nice burn to it( brown). Is there a way I can test with my DVOM?

Customer: replied 3 years ago.
I already replied
Expert:  Mike S. replied 3 years ago.

What does the label under the hood say the timing should be and why do you say you purposely have it set to 3 degrees? Are you getting a good exhaust flow? No clogged converters, crushed muffler, etc?

If you have a vacuum gauge, try this. Remove everything and plug all the holes in the valve covers, then remove the dipstick and place the vacuum gauge on the dipstick tube. It should show pressure, if it shows vacuum you have an internal vacuum leak.

Customer: replied 3 years ago.
Do you want me to plug the ports on the EGR,EGR solenoid, evapration cannister line, MAP. PCV port on carb or just the valve covers? You did not comment on the O2 sensor as I asked.
Expert:  Mike S. replied 3 years ago.

If there is only one and that looks good no reason to suspect it isn't doing it's job not allowing too rich or too lean conditions from fouling it or burning it up.

I have to ask if you reved it up and kept it constant there at the rpm you said it was missing and check for the misfire then? Although you could guess by looking at the plugs or use both to confirm both.

Before finding the problem, first you have to recreate it first so you can test. If not the problem may be hidden. If it's truly intermittent even at a specific rpm, then it may be wiring, so wiggle tests may help to recreate it.

Since of so many fuel injectors a faulty injector would also cause a miss on 1 or more cylinders and could be intermittent due to wiring because they are electric driven by grounds through the ecm/pcm.

I heard of anytime you have an engine performance problem, hook up everything you got, but an oscilloscope but a cylinder balance test was all that was good for. Hook up a tach, timing light, fuel pressure gauge, vacuum gauge, code reader or since OBDI that key chain jumper, go around and inspect the exhaust and take it for a ride or not. Keep an eye on everything looking for a fault while reving up and down, wiggling wires, spraying starting fluid around checking for leaks, covering the air horn making it stall, making it run rich.

By now with the misfiring the plug or plugs taht have been misfiring are probably fouled by now but maybe not if still intermittent. Nonetheless, they should be inspected to look for rich or lean mixtures, oil or carbon, etc and might as well replace them.

If you ever find the misfire, swap a spark plug wire and check again. If still misses there and didn't move than either disconnect the fuel injector wires and do a balance test like that to see if it makes a change. If after that and nothing measure camshaft lobe lift on both valves.

Customer: replied 3 years ago.
It seems to me that we are at a stalemate. I will recheck the plugs(,By the way I did check for a vacuum leak again and found nothing.) I just put my DVOM on to monitor the miss. It will drop 40 to 80 RPM routinely and about 300 RPM on a hard miss. I took it up to aboout 1200-1300 rpm before i did not notice any noticible miss. As you know DVOM flucuate pretty easy.
Customer: replied 3 years ago.
I wonder if perhaps we have a lean mixture problem. I sprayed propane into the regulator air inlet and it seemed to smooth out a little. I am baffled.
Expert:  Mike S. replied 3 years ago.
Different fuels make it run different. To see what it would de normally shoot some right into the throttle body. So when you test for a vacuum leak instead of waiting for the rpm to drop it would instead rise. Maybe it is too lean, you would think propane burns like a heating fuel, so it should make it richer. Or possibly because it is a light gas does something else. I know axcelyene used to do the same thing. Yeah, I wanted you to check for an internal vacuum leak as well as outside. You would norammly unhook anything on the valve covers that could leak vacuum and plug them all up and leave just the 1 for the gauge. Isn't that or doesn't that have an idle bypass or something similar that would get clogged from time to time. I know you said you rebuilt the tbi, did you run wire through all the holes and also blow out with air? Look at teh spray pattern on both injectors, I like to use the timing light, kinda freezes a snapshot to get a good bright look at. Look for any drips.
Expert:  Mike S. replied 3 years ago.

Dropping 40 to 80 RPM routinely is probably 1 cylinder misfiring and the 300 rpm is most likely 2.

If you knew which ones you could move the plugs and wires around and follow the miss or if it stays right there it's not a plug or wire.

Customer: replied 3 years ago.
I will do the cylinder balance test again and the internal vacumm test and let you know the results.Will you be around in an hour or two or do you work tommorow
Expert:  Mike S. replied 3 years ago.
It's actually getting late now. I'm on as early as 7 most of the time in the morning.
Customer: replied 3 years ago.
ROGER THAT. aGAIN THANKS FOR THE HELP AND INSIGHT. sEE YOU IN THE am
Expert:  Mike S. replied 3 years ago.
Ok, I awake.
Customer: replied 3 years ago.
sorry about the delay. Spouse has control over weekends. Since we last talked, I went ahead and wasted my money and installed a new O2 sensor-no change. I also changed all the plugs -no change althoug the ones I pulled out were clean and nice tan brown color. The big news is I pulled the new distrubutor and took it back and exchanged it. The truck runs really smooth now and I checked each plug with my Fluke and the RPM variation is much less 10 or so and that is common for a DVOM. What do yuo think at this point?
Customer: replied 3 years ago.
sorry about the delay. Spouse has control over weekends. Since we last talked, I went ahead and wasted my money and installed a new O2 sensor-no change. I also changed all the plugs -no change althoug the ones I pulled out were clean and nice tan brown color. The big news is I pulled the new distrubutor and took it back and exchanged it. The truck runs really smooth now and I checked each plug with my Fluke and the RPM variation is much less 10 or so and that is common for a DVOM. What do yuo think at this point?
Customer: replied 3 years ago.
sorry about the delay. Spouse has control over weekends. Since we last talked, I went ahead and wasted my money and installed a new O2 sensor-no change. I also changed all the plugs -no change althoug the ones I pulled out were clean and nice tan brown color. The big news is I pulled the new distrubutor and took it back and exchanged it. The truck runs really smooth now and I checked each plug with my Fluke and the RPM variation is much less 10 or so and that is common for a DVOM. What do yuo think at this point?
Expert:  Mike S. replied 3 years ago.
Well, either that distributor was faulty or it was a tooth off. Everything ok now?
Customer: replied 3 years ago.
How would it being off a tooth matter if you re-timed it to #1 cylinder . I think I jumped the gun in my last answer. The The dang thing runs good one period and then goofy and back to smooth again. No, it is rough upon starting struggles, and then smooths out after a couple of minutes It has never done that before. But it is better than when we started. I will have to drive it some to really answer your question, "is it ok now". The reason being it seems to slightly miss in gear 525-550 RPM. It seems if I raise the idle something lowers it??? By the way I did the internal pressure test as you suggested and we have slight positive pressure but NO vacuum. I have been busy changing the dang lower water hose as it started leaking. Do I have the luck or what. Talk to you in the AM.
Expert:  Mike S. replied 3 years ago.

A distributor can looked timed but, if you were to look down at the pickup coil and the tooth on the shaft they may be off when the rotor is facing #1 on the tower. What this causes is the distributor to fire in between towers. I didn't say it was I just said it could have been.

The manual I have says refer to the underhood label for idle speed but 550 rpm seems a bit low, as a matter of fact the 1988 and 89 models they do list the idle or and they have it listed at 700 rpm.

Ok, a pressure during that test is a good thing, that passes.

If you have a vacuum gauge see how much intake manifold vacuum you have at idle. It should be at least 17 inches and 19 inches would be great.

Look at the fuel injectors while it's idling and look for any fuel dripping, there should not be any drips.

If it does have a miss at idle you may at least be able to tell which bank it is if it is a dualk exhaust by going around to each tailpipe and put the palm of your hand about 6 inches away from the exhaust gas and feel and look and listen for any spitting and sputtering. Plus then smell your hand and smell if you smell gas or if the exhaust burns your eyes, that is too lean, like gas too rich.

I don't know if I gave you one yet or not, but for the testing procedure for the idle air control valve, click here.

Expert:  Mike S. replied 3 years ago.

Also, if you have a scan tool you can check if the IAC circuits are clooged or if there may be a vacuum leak.

graphic

graphic

Customer: replied 3 years ago.
Ido not have a scan tool but I will see if I can do it with my DVOM. I cannot read the idle on my lable as it is faded and the GM manual I have says cannot be adjusted.. I had to drill out the idle screw anyway as it is set at the factory and not meant to be adjusted. I wii set it to 600 in gear anyway when it warms up. As for the engine vacuum test I have no direct manifold ports to check it. Do you know of any
Expert:  Mike S. replied 3 years ago.
Do you have the brake booster hose where you can get to it?
Customer: replied 3 years ago.
yes I have a brake booster hose. I wii try that. I am going to attempt to measure counts from my IAC with my DVOM.Where would I put the leads.
Customer: replied 3 years ago.
We have a steady 21 inches of vacuum from the brake booster
Expert:  Mike S. replied 3 years ago.
21 inches of vacuum is great. On that IAC counts with a DVOM, I have no idea even if you can do it. Perhaps you should just take it off and clean it off and blow out the passages with air.
Mike S., Chevy Mechanic
Category: Chevy
Satisfied Customers: 5178
Experience: ASE Certified Master Technician
Mike S. and 10 other Chevy Specialists are ready to help you
Customer: replied 3 years ago.
Remember it is a new IAC and I rebuilt the TBI fuel regulator. I found in the chevy manual for my year and model etc- it says 525 for RPM in gear for auto and 60 counts IAC. But does not tell me where to put the leads. I will keep researching
Expert:  Mike S. replied 3 years ago.
Ok, cause I don't see that.
Customer: replied 3 years ago.
I cannot find it either. I wii do as you sugested and remove it and blow out the ports. I was wrong on that IAC count for my engine. The manual says 5-30 counts.
Expert:  Mike S. replied 3 years ago.
Any news?
Customer: replied 3 years ago.
I drove the truck for about 60 miles yesterday. It still has a miss or stumble or is hunting but not as bad as before. I have run out of money and time and we haven't solved the problem. Lets split the difference now and when I return in 2 weeks we can start again. Plus I can put a few more miles on it. Sound fair?
Expert:  Mike S. replied 3 years ago.
ok

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