91 GMC rough idle

[Kevcules]

I have a 91 GMC Sierrra truck with a 305 engine with 260,000 km's.
It has been acting up for some time. In the last 8 months it will idle so close to stalling ,all the lights on the dash will come on then go out.It never does stall though. I can't say it always idles the same. Usually when I hammer the throttle a little,it acts worse. There are no trouble codes.It doesn't idle high ,so I suspect no vacuum leaks. EGR replaced last year and working. Distributor shaft replaced two years ago.(helped with spark knock) The coil is about 12 years old. It's kept pretty tuned up ,and I use injector cleaner often.
Can someone tell me what it can't be and what it can?

Here's what I think it could be..

Stretched timing chain
Worn cam and lifters
Worn out fuel injectors
Compression (different cylinder to cylinder)

I don't believe it's electrical because I have no codes, but not convinced it isn't by the different way it acts.
Any info is welcome.

Thanks

Kevin

[gremlin96]

Worn cam and timing chain.

[j cAT]

idle issues are vacuum related ...with a vacuum guage check for leaks/proper vacuum ...which should be 18inches at the normal idle RPM...

hoses and intake gaskets ...then with a vacuum guage you can observe the vacuum and then get a good indication of the engines issues with have the vacuum reading changes ...

see my posting chart " engine vacuum troubleshooting " ...caprice posting ...................

[Kevcules]

I had checked the vacuum with a gauge a while back and it read 20 hg I think.(as per my manual) It was steady so I don't think I have a vacuum leak.
I think I'll check my "cam lobe lift" at the rockers. Maybe the cam is shot. Can you tell the condition of the cam/crank timing "chain" other than removing the cover and looking at it?

Kevin

[j cAT]

Originally Posted by Kevcules

I had checked the vacuum with a gauge a while back and it read 20 hg I think.(as per my manual) It was steady so I don't think I have a vacuum leak.
I think I'll check my "cam lobe lift" at the rockers. Maybe the cam is shot. Can you tell the condition of the cam/crank timing "chain" other than removing the cover and looking at it?

Kevin

the vacuum if now steady and at 20 inches looks good ..also indicates to me that this is still a good running engine with good compression because you do not get that kind of reading with worn engine componets....

I suggest you get to check out the grounding points and all the electrical connections .....also any fuseable links that are corroded ..

then this to could be a worn ignition switch contacts...

[Kevcules]

Hey guys

I went ahead and cleaned all the electrical connections I could see (there aren't that many) and bought a new ignition coil (old one was 14 yrs old) . I also cleaned and checked the distributor cap and rotor. There was alot of carbon on the inside cap terminals so I scraped it off and put on some dielectric grease.The plugs and wires are good.I checked the vacuum again and it's still steady at 20 hg.
It works good but still stumbles at idle ,not consistent stumble either. Any other ideas?

Kevin

[MT-2500]

Originally Posted by Kevcules

Hey guys

I went ahead and cleaned all the electrical connections I could see (there aren't that many) and bought a new ignition coil (old one was 14 yrs old) . I also cleaned and checked the distributor cap and rotor. There was alot of carbon on the inside cap terminals so I scraped it off and put on some dielectric grease.The plugs and wires are good.I checked the vacuum again and it's still steady at 20 hg.
It works good but still stumbles at idle ,not consistent stumble either. Any other ideas?

Kevin

If engine vacuum is good and no engine miss the PCM/IAC valve should control idle speed.

Any codes or check engine lights?

[kahjdh]

Fuel pressure? not enough volume?

[j cAT]

Originally Posted by Kevcules

Hey guys

I went ahead and cleaned all the electrical connections I could see (there aren't that many) and bought a new ignition coil (old one was 14 yrs old) . I also cleaned and checked the distributor cap and rotor. There was alot of carbon on the inside cap terminals so I scraped it off and put on some dielectric grease.The plugs and wires are good.I checked the vacuum again and it's still steady at 20 hg.
It works good but still stumbles at idle ,not consistent stumble either. Any other ideas?

Kevin

why is there carbon on the inside of the distributor cap ? this is telling me this is where your problem is ...make sure that the distributor is good that the bearing is good , rotor does not move around too much ...or the parts are not to correct spec....

[Kevcules]

No trouble codes or engine light showing. There is a definite miss or stumble to the engine at idle.Not always as noticeable. The idle speed does seem low to me when in drive,but in park ,fine.

I replaced the fuel pump and tank 2 years ago.I never checked fuel pressure but assumed it was good as there is sufficient power.Wouldn't I notice lack of fuel pressure or volume at higher engine revolutions?

There has always been carbon traces on the inside terminals of the distributor cap since I've owned it. I thought this was normal.(needed to be cleaned and or cap replaced)

I replaced the distributor shaft 2 years ago and there is little play in the rotor when I try to turn it.

Any more ideas are welcome.

Kevin

[MT-2500]

If the only problem is idle work on and check IAC valve sndIAC valve control.
The PCM and iac valve should hold idle upto specs.
Also make sure pcv valve is ingood shape.

What RPM is it holding in park and drive?

[Kevcules]

I replaced the IAC valve already with very little change.

I have scanned the PCM a few times now and have no pending or active DTC's . I have only used the onboard diagnostic tool lately,that would still pick up any codes right?

My PCV valve is still clicking ,maybe I'll replace it anyway.

I just thought of the RPM thing the other day. I don't have a tach so I'm wondering how I can get that info.Maybe I can lend a tach and wire into the proper wires.I'll check into it. I do suspect it's not as high as it should be in "drive" anyway. In park it sounds fine at a little higher RPM.

Thanks

Kevin

[Mango™]

what about the battery, could be week and not able to keep the truck going. Battery light on dash could be out.

[Kevcules]

Thanks for the idea....
The battery was replaced a year ago (cranks the engine fine) and all my dash lights come on when I start the truck.

Kevin

[Kevcules]

Still here ... I still didn't get the rpm's of my engine but I would say it's just a tad on the low side when in drive,very close to normal.
A mechanic friend of mine mentioned EGR valve and fuel pressure regulator.My EGR is fairly new and will stall the engine when compressed and the engine is hot..
He said I would find the regulator in the throttle body between the 2 injectors?? Correct?? Also ,what kind of adaptor do I need to test my fuel pressure? I don't have a screw on area for the gauge.
I changed the fuel pump less than 3 years ago so I hope it's still good. This regulator makes sense where the idle comes and goes. Any more thoughts on my rough inconsistent idle ??

Kevin