98 GMC ignition problem

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#1
engine misses at low rpms. mostly noticeable at 45-50 mph after torque converter lockup, but even does it in 3rd gear. at times it even starts a momentary nonstop miss similar to driving on a rumble strip. engine has 147,000 miles. new coil, cap, rotor, plugs, and wires. it never throws any trouble codes when it is acting up. this occurs only after engine has warmed up - cold engine runs great. a local shop drove it with a scanner hooked up to it while it was acting up and couldn't figure anything out either. . . . any ideas where to look next? not sure what it is called, but the plate that the rotor attaches to is quite corroded. i think that's all the info i can give on it. thanks for any help.
 
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#2
not sure which engine you have? some thoughts may not apply- but more info is always better


never throws codes. is the check engine light ever on (thinking specific codes)
stock no Mods ?
Did you change ign module?
MOre important than you think. What brand parts did you use??

some ideas:
Check the EGR, post which valve Neg , linear, EGR commanded only when warmed up,
check for a vacuum leak at the intake maniflod
Fuel pressure and bleed test- losing pressure -internal leak
 
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350 engine. had ign module checked and it tested good. for some reason i still suspect it. . . . i tried getting it in there while still hot, but i think it may have cooled off before they got to testing it. engine is bone stock. i installed a new distributor about 2 years ago and kept getting a P1345 code, but i just turned the base a bit until the code disappeared and kept driving it that way. i had the shop set the camshaft retard with the scanner recently hoping that was part of the problem. had to keep replacing cap and rotor about every 10,000 miles due to corrosion. finally just drilled out the air vent holes in the distributor base. the P1345 code is the only one it has shown in over 2 years though. . . . i use NAPA parts - no discount store stuff. i understand i will need to take the EGR to the shop for testing, but i do not understand "which valve Neg" what does that mean?
 
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#4
different types of EGR. negative backpressure, linear, and a third I cannot remember... some are easy and some require a scanner

That is the problem with testing the ICM...buy the time you get out and to store they have cooled
 
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#6
If your breaker plate is all corroded, ever think about cleaning or replacing it? That serves as a ground, least the ground contact points have to be clean. Wire brushing with silicone grease to help retard the rust. Or get it replated, they do that in China now, EPA killed that in the USA.
 
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Breaker plate? is that the term for the plate where the rotor attaches? i cleaned everywhere i could get to (top and outside) but couldn't get the underside or inside. is there a way to remove that to replace it or clean it?
 
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#8
Start by marking the relationship of the cam gear with the shaft, want to put it together exactly how you took it apart, knocking out the roll pin, and pulling the shaft out of the top end. I like to press in new bushings and ream them while I am at it. Ground is also between the aluminum base of the distributor and cast iron manifold, dissimilar metals that somebody forgot there is a rule against that also has to be cleaned and silicone greased to prevent electrolysis, but works well past the warranty period. Are you beyond the warranty period.

With the shaft out, the rest is easy.

I have done an all electronic about four years ago, and a centrifugal about two years ago so not an everyday occurrence for me, but when in my hand, all comes clear again. Male terminals are easy to clean and polish, but the females have to be removed first from their housings for cleaning or replacement. And always find flux joints on the trigger coil that expand and break contact, girls were never allowed to burn off that flux that would take an extra second. But back in 1972, an entire HEI assembly with the spark plug leads attached had to be provided from Delco to GM for six bucks. Delco couldn't do that anymore and today, there is no Delco.
 
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#9
okay, here's the update. took EGR off and cleaned it up - it had a bit of carbon buildup that didn't seem too excessive to me. fuel pressure is at 60psi with key on engine off. drops to 54psi at idle (cold). at warm idle, the needle jumps between 54 and 58psi. at warm idle in gear, needle flutters between 52 and 58psi. does anything here sound wrong?
 

Gus

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#10
You need a scan tool to pinpoint which cylinder is misfiring...then if spark and compression and the plug is ok, you concentrate on the injector...
 
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#11
knucklehead said:
okay, here's the update. took EGR off and cleaned it up - it had a bit of carbon buildup that didn't seem too excessive to me. fuel pressure is at 60psi with key on engine off. drops to 54psi at idle (cold). at warm idle, the needle jumps between 54 and 58psi. at warm idle in gear, needle flutters between 52 and 58psi. does anything here sound wrong?
That fuel pressure is not good.
You need 64-65 lbs of pressure on a cold start or the injectors wiil no squirt.

The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold or hot and fuel pressure testing guide lines.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.
 
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#12
well i never would have thought it but i noticed that the tranny fluid level looked a little bit high, so i siphoned a bit out. stuff was colored like a black cherry! drained it and replaced the filter and now it runs great. sure felt like an engine miss to me. oh well, it's fixed now. thanks everybody for all your help!