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idle miss/engine dies

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Anonymous Poster

Jun 13, 2009, 10:24 PM

Post #1 of 12 (1593 views)
idle miss/engine dies Sign In

I have a 1988 GMC K1500 P/U with the 5.7 liter engine and 184,000 and some change. I have noticed lately that I have a low idle miss when I am at a stop sign or light. The last week or so, the engine has died a few times at this idle speed mostly when shifting from drive to reverse and back but a few times at the stops as well. I am thinking the miss is one cylinder only. The engine does not miss at any RPM aside from idle. When accelerating and driving it runs smoothly. The fuel filter can be an issue on this truck if not replaced regularly which I did the other day. No help. It still has the low idle miss but has not died on me.

I have not had a tune up for some time but seem to remember no issues with plugs the last time it was done that might have indicated a cylinder was not firing properly. I believe it has been about two years since the last tune up which included new plugs of course. Perhaps 20,000 miles since the last tune up.

Any ideas on which direction to go with this would be appreciated. Thank you.


Loren Champlain Sr
Veteran / Moderator

Jun 14, 2009, 12:16 PM

Post #2 of 12 (1586 views)
Re: idle miss/engine dies [In reply to] Sign In

Jason; If it's been 20K since the last tune, it's time. Wink If you can do this yourself, it won't be too expensive. Don't overlook the distributor cap, rotor, and wires. If you can afford it, just replace them. I believe that this is the TBI type injection? A very common problem is the TBI base gasket sucking in on the back side and causing a vacuum leak. Also, remove and clean the IAC while you are doing the tune. There are a lot of reasons, of course, that can cause the symptoms that you describe, but need to start off with the basics that we know it needs.
Are you getting any lifter/rocker arm noise at idle? Any backfire under accel? ect, ect, ect.
SW Washington

Jason T
Anonymous Poster

Jul 2, 2009, 7:15 PM

Post #3 of 12 (1522 views)
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Thank you for your reply. I have completed the tune up;plugs, wires, dist. cap etc. Plugs were gapped properly and I did the wires one at a time so as to not mix one up. No help. Still have the low idle miss and the engine quits at low idle occasionally. It starts right back up just fine and off I go.

Yes, this is the TBI system. The first year it was used since I think GM changed everything about their pickups after the 87 model year. There is no lifter or rocker arm noise; this motor runs as smooth as silk,burns no oil, gets good mileage for what it is and passes emissions every year. No backfires on acceleration or any other time.

I assume the intake manifold has to come off to replace the TBI gasket? That is something I do not think I can handle myself; lack of the proper tools. What is the IAC?

Any other suggestions on what might be causing this miss and the engine stop issue?

The plugs all looked fairly good as I took them out except one was a bit darker than the others. What about a bad injector? I can not think of any other clues to give you. Burned exhaust valve on the cylinder that had the darker looking plug when I pulled it out? I am talking to myself now about things that I know are beyond my capability to repair myself.

Thank your for any other ideas you may have. Jason

Loren Champlain Sr
Veteran / Moderator

Jul 3, 2009, 1:38 PM

Post #4 of 12 (1512 views)
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Jason; >>Still have the low idle miss<< >>this motor runs as smooth as silk<<
Okay, now, I'm confused. Smile The IAC (idle air control) is an electric solenoid that screws into the TBI unit. You'll see an electrical connector going to it. Buy some throttle body cleaner and a tooth brush. With the engine off, grab the throttle linkage by hand and open the throttle blades wide open. Spray it down with the cleaner, use the tooth brush to thouroughly clean the throttle plates and the bore of the TBI unit. Remove the IAC and clean it, also. You can 'rinse' both with carb cleaner, but remember that the more you spray in there, the harder it's going to be to start the engine. Put the air cleaner assy. back on before trying to start. Just crank it. Don't use the throttle at all until it starts. Once started and has returned to 'normal' idle speed, you can carefully (carb cleaner is flammable) spray some of the carb cleaner around the base of the TBI where it connects to the intake manifold. If the rpm changes when you do that, then the gasket is probably bad, sucking in air. (this is usually at the rear of the TBI on the driver's side, typically). Very easy to remove the TBI and replace the gasket. No different than removing a carburetor. If you need further help, don't hesitate to ask. Have a great weekend.
SW Washington

Anonymous Poster

Jul 14, 2009, 6:13 PM

Post #5 of 12 (1467 views)
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Loren, thank you for your reply. Sorry for confusing you. You asked earlier about lifter/rocker arm noise at idle and backfires on acceleration. What I meant about running as smooth as silk is the engine itself has no abnormal internal noises as it runs, at idle, acceleration or when I put a load on it. The motor seems great to me for such high mileage. It does however have this irritating miss when at a stop sign. The miss feels like only one cylinder and is not constant. It will miss a time or two and then run smoothly for a few seconds and then start to miss again and then stop and run smoothly. Sometimes the engine will die. It will immediately start back up again and off I go.

I will try your suggestions and see what happens. About the TBI and its removal. Can I take out the fasteners(bolts or whatever they are) and lift the TBI body up to replace the gasket and leave all the linkages in place? Or does one have to disconnect the linkages and unbolt the TBI to replace the gasket.? I have never pulled a carb before but have always been intimidated by all the bolts,screws and linkages that are there while I have looked at them.

My thanks for your help. Since I found this forum, your help has increased my knowledge, boosted my confidence and quite frankly saved me a bundle with things I have repaired myself. After I figure out this miss thing or give up on it for now, I do have another issue I would like to have your help with. Same vehicle. Again, thank you.


Veteran / Moderator

Jul 14, 2009, 6:59 PM

Post #6 of 12 (1466 views)
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Jason.....I'm not gonna derail Lorens thread here, cuz he is giving you some real good advise on narrowing down your problem but just disconnect the vacuum line off the EGR valve & take the truck for a spin and see if that makes any difference.........


Loren Champlain Sr
Veteran / Moderator

Jul 15, 2009, 1:57 PM

Post #7 of 12 (1459 views)
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Sidom; Excellent idea; Had forgotten about the pickup coil issues with these. Or, simply, EGR sticking open....
Getting old sucks. LOL.
Jason; Yes, you can unbolt it, raise it and replace the gasket, but it's going to be tough getting the base of the TBI and the intake manifold surfaces clean without actually removing it.
SW Washington

Anonymous Poster

Jul 19, 2009, 5:08 PM

Post #8 of 12 (1451 views)
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Sidom, Thank you. I'm sure Loren's ok with you jumping in. I do not know where the EGR is so I can disconnect that vacuum line and see if the miss disappears. Can you explain about that? And if disconnecting the vac. line cures the miss, then what? Leave it disconnnected or does that lead me to repair, replace a part? Thanks, Jason

Anonymous Poster

Jul 19, 2009, 5:14 PM

Post #9 of 12 (1450 views)
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Loren, ok, I'd not thought of having to clean both surfaces to replace the gasket; how dumb, I have replaced gaskets before and know how that goes.

I have asked Sidom where that vacuum line is as well as the EGR valve since I don't know. I am supposing if the miss disappears once that line is disconnected that means something. The EGR valve being stuck? What did you mean about the pickup coil? How does all this fit together? Thanks much, Jason

Loren Champlain Sr
Veteran / Moderator

Jul 20, 2009, 1:21 PM

Post #10 of 12 (1444 views)
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Jason; Disregard the comment about the pick-up coil; That was on the older distributors with a vacuum advance. This pic is of the EGR valve. The vacuum line going to it should only have vacuum above 2000 rpm. (ported vacuum). If it has vacuum going to it at idle (manifold vacuum) that will cause it to open and act like a severe vacuum leak.This is the 'underside' of the valve. Yours may look different, as this is a 'generic' pic. Notice the square hole and the round one with the pintle in the center. When the valve is closed, you should be able to pour a liquid in the square hole and it shouldn't leak through into the round one. If liquid leaks through, you can try cleaning the pintle area. Sometimes, a chunk of carbon will hold it open.
SW Washington

Veteran / Moderator

Jul 20, 2009, 4:49 PM

Post #11 of 12 (1439 views)
Re: idle miss/engine dies [In reply to] Sign In

Well...Loren has it laid out pretty good as usual. Like Loren refered to, it's basically an emission control device use to keep combustion temps down using exhaust gases. It's suppose to kick in ad mid & high rpms. The ones I've run into have a weak spring in the valve allowing the pintle to open to soon (basically @ idle) & cause a low speed miss & some times dying...

75% of the ones I diag wind up driving around with a bolt in the vac line about a 1/4 of the people fix the problem.....go figure Angelic


Anonymous Poster

Jul 21, 2009, 6:57 PM

Post #12 of 12 (1434 views)
Re: idle miss/engine dies [In reply to] Sign In

Loren and Sidom, Thanks to you both, I think I have the picture now except, where is it on my 350 engine?

idle miss/engine dies

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