*GMMerlin*89 GMC Rough Idle and Stalling


jtjsys
10-11-2004, 07:17 PM
I am having the problem that alot of these guys are having with stalling, etc. Have replaced EGR, temp sensor (manifold), IAC, ECM, MAP sensor, injectors/reg, plugs wires cap and rotor. Have checked throttle position sensor, ignition module. Still nothing has made any difference in my problem. It looks like the fuel flow from the injectors into the TB is interrupted slightly as the engine runs rough or dies suddenly.

Saw your advice about pickup coil and and don't know where that's located. Also saw something about a fuel pump relay and don't know where that might be.

I suspect the fuel pump may be weakening, but don't know how to test it. Is there a gauge kit I can buy to check fuel pressure? What PSI is correct?

jtjsys
10-12-2004, 08:03 PM
Well, after all of that... when I went to change the pulse coil inside the distributor, I found that the upper shaft bushing was worn out, causing a whole array of ignition problems, and ultimately causing my idle and run problems. Replaced the distributor and she runs like a top. Hope this helps keep someone else from chasing what looked like an emission system or fuel system problem.

astralepic
01-04-2005, 09:04 PM
Hey jtjsys, you still around? I'd like to talk more. I'm having same issue and would be interested to know what all you replaced/checked. My distributer appears fine or atleast did a few days ago.

GMMerlin
01-04-2005, 09:31 PM
Hey jtjsys, you still around? I'd like to talk more. I'm having same issue and would be interested to know what all you replaced/checked. My distributer appears fine or atleast did a few days ago.

Looks like someone called me out while I was gone for a while.
Anywho, how about posting a description of whats going on with your truck and we will see if we can get it straightened out.
I have more time to lurk around now :evillol:

astralepic
01-05-2005, 09:08 AM
Hey GMGerlin, you replied in the post http://www.automotiveforums.com/vbulletin/showthread.php?t=341505. I'm still working on my issue, but the IAC doesn't appeared to have helped. The more I read up on O2 sensors, the more I think that may be atleast a portion of my problem. I have never replaced it and the truck has 230,000 on it. Referencing back to the post above, I have a 1993 GMC Sierra, 2WD, 350 TBI. Just recently the motor started to idle rough while at a stop. It runs fine under load, it's just at an idle.

I started with thinking I was having a vacuum issue, but was unable to find any vacuum leaks so I purchased a code reader even though the SES light was not on. No codes were logged and it began to get worse so I took it to a local shop. Nick, the guy that worked on it told me that a plug was filed out and that it was burning oil really bad. Long story short, he could not answer my questions about which cylinder it was and etc. The guy even went on to say it was ready to blow, but he knew of someone that would probably buy it. Needless to say, I went to get the truck and it appears all they did was plug it up to diag and received nothing just as I did. Got the truck and decided to look further myself. I've put over 200 miles on it since picking it up and it's not burning oil at all or atleast not enough to notice at present.

Once again referencing the post above, I have replaced the IAC, but I'm not sure it is the correct overall length. Does this matter? The instructions told of how to reset the control, but I didn't know if the reset would handle an incorrect length of the pintle. Do you think this is something to be concerned with?

I have a new O2 sensor and plugs I plan to put in this weekend. I really can't complain about my truck because I got my money's worth, but I'm not ready to give up on her.

I appreciate it man! If you have any ideas or need more information, please let me know. Thanks again!

GMMerlin
01-05-2005, 11:58 AM
Hey GMGerlin, you replied in the post http://www.automotiveforums.com/vbulletin/showthread.php?t=341505. I'm still working on my issue, but the IAC doesn't appeared to have helped. The more I read up on O2 sensors, the more I think that may be atleast a portion of my problem. I have never replaced it and the truck has 230,000 on it. Referencing back to the post above, I have a 1993 GMC Sierra, 2WD, 350 TBI. Just recently the motor started to idle rough while at a stop. It runs fine under load, it's just at an idle.

I started with thinking I was having a vacuum issue, but was unable to find any vacuum leaks so I purchased a code reader even though the SES light was not on. No codes were logged and it began to get worse so I took it to a local shop. Nick, the guy that worked on it told me that a plug was filed out and that it was burning oil really bad. Long story short, he could not answer my questions about which cylinder it was and etc. The guy even went on to say it was ready to blow, but he knew of someone that would probably buy it. Needless to say, I went to get the truck and it appears all they did was plug it up to diag and received nothing just as I did. Got the truck and decided to look further myself. I've put over 200 miles on it since picking it up and it's not burning oil at all or atleast not enough to notice at present.

Once again referencing the post above, I have replaced the IAC, but I'm not sure it is the correct overall length. Does this matter? The instructions told of how to reset the control, but I didn't know if the reset would handle an incorrect length of the pintle. Do you think this is something to be concerned with?

I have a new O2 sensor and plugs I plan to put in this weekend. I really can't complain about my truck because I got my money's worth, but I'm not ready to give up on her.

I appreciate it man! If you have any ideas or need more information, please let me know. Thanks again!

OK lets start with the basics and move from there.
You had a guy look at it and he said it was fouling plugs, but couldn't tell you which one?
What engine do you have?
You say this is a rough idle problem..how bad?
If this engine has 230k on it the first thing I would do is perform a compression test..this will give you a basic idea of what condition the cylinders are in..all the cylinders should be within 10% of each other with none below 100psi
Also a vacum gage hooked up while the engine is idling will give you some hints on what is going on.
is the vacum high and steady (15-20) or is it low and steady(indicates a vacum leak) does the vacum bounce around (misfire or internal angine problem)
These 2 tools will tell you more than anything else.

As far as the IAC valve goes, it moves inside the bore, so the length will change..the most important thing is to make sure it is depressed most of the way in before installing it.
Try this and get back with us

astralepic
01-06-2005, 05:15 PM
Thanks GMMerlin! I will get a compression tool and check it over the weekend. I went ahead and replaced the O2 sensor last night and still no change. As for a vacuum gauge, can it be anywhere in line or does it have to be connected at a specific place?

To answer all of your questions, the gentlemen at the local shop did tell me that it was fouling a plug, but could not tell me which cylinder. When I picked it up, it was obvious they never had a wrench on it. In addition he stated they removed the throttle body and cleaned it, but it still has gunk caked on the outside of it. On top of all of this, the guy had the nerve to tell me he knew of someone that would buy the truck. That is when I started to get suspicious and decided to pick it up. I have the 5.7 liter, 350 with TBI. It has all of the obvious luxuries, AC, PS and etc. The truck is a 1993 GMC Sierra, short bed, 2WD.

The rough idle really isn't that bad. It's not bad enough to kill the motor, but when sitting at a traffic light, it will lug the motor down and then back up to idle, down, back up and just goes through that sequence but never dies. It does not do it at all when pressing the accelerator even the smallest amount. When driving there is no lack of power or response. I really think the motor is in fine shape, but I'm willing to do what is needed to get a handle on this. It has never burned a drop of oil and since picking it up from the service center, I have put 300 miles on it and no oil has been burned or not enough to notice it on the dip stick. Did I mention the rough idle only seems to be once the motor gets warm???

I appreciate it man!

sechracer
01-06-2005, 07:50 PM
possibly a tps sensor?

astralepic
01-06-2005, 07:57 PM
My experience with TPS has pointed to very harsh shifting. It has been replaced once, but we'll keep it in mind.

astralepic
01-08-2005, 07:47 PM
Ok GMMerlin, here's what I've got. I performed the compression test as you advised and here are the results. I will format this in firing order. 1) 160, 2) 162, 3) 170, 4) 171, 5) 165, 6) 160, 7) 165, 8)172. I consistently used 5 rotations of the motor to produce these results and ran each cylinder multiple times. I used a video camera as I was performing this by myself and it appears the psi is building consistently with each stroke. I ran the test 3 times per cylinder and without posting all results, they were always close. Only a difference of 1 to 2 psi per cylinder with each test. What are you thoughts here?

As far as the vacuum goes, I need to know where I place the gauge. Is it anywhere in line or is there a particular point that I need to place it?

As always, I appreciate your input and help.

As a sidebar, I went ahead and replaced all plugs, wires, coil, module, rotor button and cap.

Thanks!

astralepic
01-08-2005, 07:49 PM
Oh and by the way, when I pulled the plugs, all looked really good. Only one had a little carbon build up on it. Nothing major at all...

astralepic
01-11-2005, 09:06 AM
GMMerlin, you still around? Anxiously waiting to hear your thoughts.

GMMerlin
01-11-2005, 02:40 PM
OK so we have determined that the internal engine is in good condition.
The vacum gauge I would hook to a port with manifold vacum..there should be one on the T/B.
When you had the cap and rotor off did you look at the inside of the dist? especially the pick up coil...was the coil in good shape or was it yellow and flaking apart?

astralepic
01-11-2005, 08:52 PM
The pickup coil did have a yellow look to it, but it appeared to be in solid shape. There was no flaking at all, but it did look a little yellow...

I will get a vacuum gauge and find the port you are referring to. Can I just disconnect the vacuum from the brake booster and test from there or does it need to directly attached to the TB?

fuzzypuppy
01-13-2005, 01:03 AM
Hello, it's been awhile since i was last on here posting, first my puter took a dump on me, then I had to dump the wife.
But my last post had to do with this very problem, I changed and or checked every sensor mentioned on this subject with the exception of the pickup coil and dist.bushing, A difference also is I cammed my truck and installed a new double roller.
the cam was an edelbrock, I dont have the spec's on hand but it was the largest I could put in and still work the puter, which as anyone knows still is not much cam, this was about 15000 ago now and i still have the rough idle, I will check the dist from top to bottom the next chance I get, (as long as the weather holds good here in so, cal.) but what I noticed as well is that my brake boost suffers, only slightly mind you, if I didn't know the truck so well and had spent my younger years building (old school) cars for street racing I would never have noticed, used to be that a vacume canister was a must in a big cam ride, in my circle of friends anyways, but my idle is a bit low and wanders more the warmer it gets, "those sensors have been checked and changed" though showed good.
I have been stumped over this one for some time now,
SPECS ARE 91, 5.7, 175.000 MI. 4L80E.
I have a set of vortecs in the garage, and now that I will be able to save rather than have someone else spend all my cash i just might be able to get those on very soon.
I can't wait!! for a time I thought the fact that I have a small crack in my driver side exaust manifold might be playing a roll in the idle problem,
but since reading all the other posts and such i doubt it makes any difference at all, my o2 sensor is down by the cat, a good 4' away or so.
Funny though that most of these trucks have the exact same problem and we all are still guessing and going through the same motions with the same results, seems we would have been able to pinpoint the problem by this point, as in the wiper motor problem hit every solder point on the board with the soldering iron, and they have been working fine every since,
just looking to get the new vortec intake and a set of shorty headers and the vortecs go on,

astralepic
01-16-2005, 08:19 PM
Hey GMMerlin, I got a vacuum gauge and hooked up to the vacuum line that went to the break booster. I figured this was good considering that it led directly to the intake manfold as you mentioned in your post. The vacuum at idle was about 20psi. It was consistent, barely moving at all. The manual that came with the gauge said to run the motor up to about 2000 rpm. I did this and the vacuum dropped to about 10psi. The motor was cold and I did not run it long enough to get it warm. It's pretty cold here right now, highs in the mid 30's. Do you think this is an issue? The manual said something was wrong with the exhaust system, but I really don't feel that's the case considering it was replaced about 3 years ago. Everything was new except the headers. New cat, muffler and all pipes.

Also, something I noticed never before was a noise under the hood. Are there bearings in the distributor? I assume so and I think one may be bad. Something around the area of the dist. was making a noise which reminded me of a bearing going bad. What are you thoughts?

As always, thanks big time for your help. We are close and I couldn't feel more confident that it's something small without your assistance.

DEEPEE
02-17-2005, 12:53 PM
I have the SAME EXACT INDENTICAL PROBLEM. Idles good cold, after a good warm up....surges sitting at a stop with rpm's fluctuating. Runs great after leaving the stop. Thought I would try and kick start this post again to see if someone knows what cause this. It's more annoying than anything especially with the magnaflow and turn downs underneath. MERLIN, can you check into this for us? I've replaced my IAC valve and checked for intake leaks. Didn't really want to start just changing parts. $$$$$$$$ Someone told me it could be the computer. $$$$$$$ Has anyone found the problem and cured it? Please let us know.

DEEPEE
02-17-2005, 03:06 PM
Forgot to mention, mine is a 5.7l 1999.

astralepic
02-18-2005, 08:45 AM
My issue was the distributor. The actual issue delt with a shim at the base of the distributor which controls advanced timing. Try it and let me know. Remember though, mine is a 93 model. I imagine there's a difference, but don't know enough about newer models. Good luck!

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