91 GMC rough idle

[j cAT]

Originally Posted by 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

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

[Kevcules]

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

[j cAT]

Originally Posted by Kevcules

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

[Kevcules]

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

[j cAT]

Originally Posted by Kevcules

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...

[Kevcules]

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

[evangelica37]

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[kahjdh]

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.

[MT-2500]

Originally Posted by Kevcules

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.

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

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.

[Big Al 14534]

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?

[MT-2500]

Originally Posted by Big Al 14534

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.

[j cAT]

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

[Kevcules]

Originally Posted by MT-2500

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.

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

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!

[MT-2500]

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

[Richard S]

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