91 GMC rough idle

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

Worn cam and timing chain.

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 ...................

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

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...

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

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?

Fuel pressure? not enough volume?

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....

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

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?

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

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

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

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

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

so how is the distributor ? did you check for bearing wear / wobble ?

My distributor feels fine ,very little movement when i move the rotor. I'm going to start checking my fuel injectors,fuel pressure and fuel regulator. I'll ohm test the injectors first...
Do you think this could be my issue?

Kevin

My distributor feels fine ,very little movement when i move the rotor. I'm going to start checking my fuel injectors,fuel pressure and fuel regulator. I'll ohm test the injectors first...
Do you think this could be my issue?

Kevin

the injectors not working correctly will mess up idle..I would try the techron injector cleaner with 3-4 oz of marvel mstery oil in the fuel ..this will clean and lube the injectors...also check your engine vacuum at idle , see if it is high and if low look for leaks ,,,vacumm lines pvc and intake gaskets

Thanks for the replies and keep them coming!

Here's an update....

I removed my distributor cap again and wanted to check for play in the rotor drive gear. I watched when the rotor started to move(i manually turned over the engine) then marked the timing damper,then rotated the other way until the rotor moved again...about 1 degree. I think this is normal for an engine with 260,000 km's...right?

Again, my distributor terminals had white carbon deposits.(cleaned them)
I replaced the fuel filter
I re-torqued my intake manifold bolts
I checked vacuum again and it's rock steady at 21 hg.(It was the same reading as before but I thought it was 22)
I found quite a bit of K&N air filter oil ,sitting on the recessed part of the injectors,blew it out,cleaned. I tried to check the resistance of the injector but couldn't get my voltmeter to read??? (never use it much, oh well)
I checked the timing and it was dead on at zero. I advanced it 1 degree.
I have a spark plug wire tester which flashes as current goes through it. I checked each wire and they all fire. One thing I did notice when I left my tester on a few wires for 1 minute ,I could see the light flash iratic,and sometimes go a bit brighter,then return to somewhat normal lower lit flashing......so..... this is telling me my fire isn't strong enough and or consistent. That's probably why the deposits are on my cap terminals and the idle is irratic!
I've already replaced the ignition coil , now I'm looking at the ignition module and pick up coil. Can you guys tell me how they are supposed to work and if this could be my issue??? Again my distributor was already changed and it doesn't have much play but I'm open to ideas.

Thanks

Thanks for the replies and keep them coming!

Here's an update....

I removed my distributor cap again and wanted to check for play in the rotor drive gear. I watched when the rotor started to move(i manually turned over the engine) then marked the timing damper,then rotated the other way until the rotor moved again...about 1 degree. I think this is normal for an engine with 260,000 km's...right?

Again, my distributor terminals had white carbon deposits.(cleaned them)
I replaced the fuel filter
I re-torqued my intake manifold bolts
I checked vacuum again and it's rock steady at 21 hg.(It was the same reading as before but I thought it was 22)
I found quite a bit of K&N air filter oil ,sitting on the recessed part of the injectors,blew it out,cleaned. I tried to check the resistance of the injector but couldn't get my voltmeter to read??? (never use it much, oh well)
I checked the timing and it was dead on at zero. I advanced it 1 degree.
I have a spark plug wire tester which flashes as current goes through it. I checked each wire and they all fire. One thing I did notice when I left my tester on a few wires for 1 minute ,I could see the light flash iratic,and sometimes go a bit brighter,then return to somewhat normal lower lit flashing......so..... this is telling me my fire isn't strong enough and or consistent. That's probably why the deposits are on my cap terminals and the idle is irratic!
I've already replaced the ignition coil , now I'm looking at the ignition module and pick up coil. Can you guys tell me how they are supposed to work and if this could be my issue??? Again my distributor was already changed and it doesn't have much play but I'm open to ideas.

Thanks

the ignition module is a possibility ....if its the original one change it ...make sure the plug wires are good ,,, arcing will damage the module...

I went to the dealership today to order the part ($105.00) but the parts guys were questioning my purchase. They advised i should talk to their head mechanic (i guess he just won an award ,"best in Canada" ) so I'll run my issue by him hopefully Friday.
By the way...for those that know,what does the ignition module do anyway?
What does the pick up coil do?
What would be giving me a weak and inconsistent spark?

Kevin

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The check engine light route wont be much help since this is an obd 1 truck. The egr vacuum solenoid on these older tbi's tend stick when they get old so try by plugging the vacuum line to the egr valve. Also ohm out the dist cap b/c i have installed brand new defects from every manufacturer on these motors so dont trust it cause its new.

I went to the dealership today to order the part ($105.00) but the parts guys were questioning my purchase. They advised i should talk to their head mechanic (i guess he just won an award ,"best in Canada" ) so I'll run my issue by him hopefully Friday.
By the way...for those that know,what does the ignition module do anyway?
What does the pick up coil do?
What would be giving me a weak and inconsistent spark?

Kevin
Why go to the dealership and pay 3 times the going rate for parts for a 20 year old truck?

You have already picked every body's brain on AF and have not taken much of there advice.:cwm27:

So let us hear what the best in Canada has to say.:rofl:

But for your information the ign module fires the coil and feeds information tho the pcm.

Weak spark can be caused by many things.
Only proper testing will answer your question on that.

I have a 93 Silverado with a 350 engine, electronic auto tranny. I have gone through the same issue (stumbling idle). Here's what I found/did-

Went through the distributor, wires, cap, rotor, plugs, coil, pickup, etc.etc.- no problems.

Checked compression- one cylinder marginally low but within GM specs.

The fuel pressure regulator was bad on the throttle body. I changed that and the fuel pump and filter to be safe (130K miles on the truck).

The ECU and all data checks out. I used a scanner that gives real-time data from the OBD-1 port (AutoX-ray 6000) so I was sure that the system sensors were sending what "appears" to be reasonable data to the ECU and it is responding.

Now, I get to the goofy part. When in drive, it stumbles erratically. No pattern, but the engine seems like it is losing spark or fuel. Fuel spray pattern from the injectors is constant. ECU data shows no "burps" on the data strings. Coil and plugs appear OK, no loss of spark. This is subjective, but the coil appears to throw a continuous spark with a neon tester. I have not hooked it to a diagnostic machine though. When in neutral or park...no stumble...constant rpm.

Someone told me it is possibly the transmission control unit. Since the transmission is electronically controlled, it appears there is some feedback to/from the ECU which may be causing this. Any ideas?

I have a 93 Silverado with a 350 engine, electronic auto tranny. I have gone through the same issue (stumbling idle). Here's what I found/did-

Went through the distributor, wires, cap, rotor, plugs, coil, pickup, etc.etc.- no problems.

Checked compression- one cylinder marginally low but within GM specs.

The fuel pressure regulator was bad on the throttle body. I changed that and the fuel pump and filter to be safe (130K miles on the truck).

The ECU and all data checks out. I used a scanner that gives real-time data from the OBD-1 port (AutoX-ray 6000) so I was sure that the system sensors were sending what "appears" to be reasonable data to the ECU and it is responding.

Now, I get to the goofy part. When in drive, it stumbles erratically. No pattern, but the engine seems like it is losing spark or fuel. Fuel spray pattern from the injectors is constant. ECU data shows no "burps" on the data strings. Coil and plugs appear OK, no loss of spark. This is subjective, but the coil appears to throw a continuous spark with a neon tester. I have not hooked it to a diagnostic machine though. When in neutral or park...no stumble...constant rpm.

Someone told me it is possibly the transmission control unit. Since the transmission is electronically controlled, it appears there is some feedback to/from the ECU which may be causing this. Any ideas?

Welcome to AF on your first post.
It is best to find the New Thread button and start a New Post on your problem.
Saves on hijacking another post.

On a 93 there was a lot of problems with the circuit board on the pcm.

Be sure to give it the old tap/twist test and see if it changes the way it is running.
Just do not use a 10 lb hammer on it a rubber one or the old fist will do .

Start a New thread an let us know how it goes.

if your issue is stumble at idle when stopped , but engine operates good otherwise this may be a bad torque converter..

Why go to the dealership and pay 3 times the going rate for parts for a 20 year old truck?

You have already picked every body's brain on AF and have not taken much of there advice.:cwm27:

So let us hear what the best in Canada has to say.:rofl:

But for your information the ign module fires the coil and feeds information tho the pcm.

Weak spark can be caused by many things.
Only proper testing will answer your question on that.

Mr MT....I take offense to your last reply.

I "have" picked the experienced brains here and am very thankful for the info. I don't have the means to test all the ideas right away ,but am trying.
I was simply trying to let the people here know that I'm trying to find someone who can test this problem for me ,and your right, I can't afford the dealership prices. I just wanted to talk to the technician. Maybe show him my distributor so he could test it.
I did find someone who is very good with troubleshooting so I'll let the friendly, helpful people here know what we find.
Thanks again!

Sorry about that.
But I call them like I read them.
Since no one hear has a crystal ball.
Proper testing is the only thing that is going to answer your questions.
Good luck

you probably have already thought of it or changed it but have you checked fuel filter.

you probably have already thought of it or changed it but have you checked fuel filter.

Just changed it last week,still have similar symptoms...thanks anyways.

Kevin

Mr MT....I take offense to your last reply.

I "have" picked the experienced brains here and am very thankful for the info. I don't have the means to test all the ideas right away ,but am trying.
I was simply trying to let the people here know that I'm trying to find someone who can test this problem for me ,and your right, I can't afford the dealership prices. I just wanted to talk to the technician. Maybe show him my distributor so he could test it.
I did find someone who is very good with troubleshooting so I'll let the friendly, helpful people here know what we find.
Thanks again!

As per your private E mail and this post.
Which does not deserve and will not get a answer from me.
But I am posting it here to show people that people like you try to bite the hand that feed them info.

Quote from private E mail from you.
I can't believe that the "advisor" on this forum would stoop so low and type some wise cracks downing myself and my country. You're supposed to lead by example and you type some bullshit unhelpful response. I was going to shit on you on the thread but you already made "yourself" look stupid!
You can take your knowledge and expertise and shove it up your ass! I don't need you to comment on any more of my questions , not that you helped anyway.
I in no way was trying to say that a canadian tech is better than the group on the forum,I welcome all information even though some of it is "not" helpful.
Grow up!
Quote.

As in post 29 I am not sorry about what said just sorry about that nothing has soaked into your brain.
Over 30 post and you still have no idea how to fix it.:rofl:
And you are lashing out at people trying to help you.:nono:

As to do not need my comments any one has the right to post back to your post.
You can take the info you get or leave it.
Good Luck

I knew you would stoop again .....so low. I sent you that message because "your" the ass , not the people here trying to help me.
You want to bring this non sense on the forum , I tried to keep if off.
Do not reply to anything I ask again and I won't waste my time with you either.
You must be some proud of your self , like I said , lead by example....you're pathetic!

I knew you would stoop again .....so low. I sent you that message because "your" the ass , not the people here trying to help me.
You want to bring this non sense on the forum , I tried to keep if off.
Do not reply to anything I ask again and I won't waste my time with you either.
You must be some proud of your self , like I said , lead by example....you're pathetic!
I am sorry but you can not tell anyone not to reply to your post.
Anyone has the right to reply to a post as long as they abide by the forum rules.
Which you are putting a hurt on and breaking right now.

If you think you have a problem take it up with the forum administrators/Moderators.

Moderators : 4

TaNK_Em, GMMerlin, Shortbus, Blue Bowtie

Success....turns out my pick up coil was bad. Thanks to everyone for their help!

Kevin

Success....turns out my pick up coil was bad. Thanks to everyone for their help!

Kevin

I would think a bad pick up coil would cause problems over the entire engine rpm range. It would not surprise me that the distributor bearing is wearing out.