1988 Chevy Silverado Intermittent rought idle

Tiny
ROMA0301
  • MEMBER
  • 1988 CHEVROLET SILVERADO
  • V8
  • 4WD
  • AUTOMATIC
  • 201,000 MILES
My truck has a intermittent rough Idle. The Idle will be steady then it will drop, get rough, speed up then return to steady. When I am on the road for 40 minutes plus at a higher rate of speed (55mph)the service engine soon lite comes on. I stop the truck, shut it off, turn it on and the light does not come back on. The service engine soon lite otherwise does not come on. When I am driving it, if I press hard on the gas for 3-4 seconds then let off it will sometimes back fire. It has a rougher idle when the truck is in gear than out of gear. When going slow (10mph) and running steady, but when I let off the rpms dip hard and it sounds like it could die but then it comes back to steady or slightly rough.
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Monday, September 20th, 2010 AT 1:01 PM

29 Replies

Tiny
JACOBANDNICKOLAS
  • EXPERT
Have you checked for vacuum leaks? Also, when the check engine light is on, you need to have the computer scanned to determine where the problem is coming from. Most parts stores will do it for free. Try that and let me know what codes you get.

Joe
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Monday, September 20th, 2010 AT 3:21 PM
Tiny
ROMA0301
  • MEMBER
I talked to the people at the auto parts store and they said that with my year of truck the computer does not store the OBD 1 codes. The light has to be on to check it. I have tried to get it to come on, but to no avail so far. I checked all my vacuum hoses and the are all connected, and not broken or cracked. Whether they are leaking or not I don't know. Now the truck has a rough Idle all the time. I thought it was the O2 sensor so I replaced that and it seemed to help a little but not much. The auto parts store suggested that it could be the EGR valve.
Any help would be greatly appreciated
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Monday, September 20th, 2010 AT 4:50 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
The EGR could play a role in the problem. Remove it and make sure it isn't carboned shut or restricted. While it is off, make sure no carbon has built up in the intake where it attaches.

Joe
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Tuesday, September 21st, 2010 AT 11:00 PM
Tiny
ROMA0301
  • MEMBER
I read almost all of the posts on rough idle, and came up with egr, or iac. I was thinking the IAC because the rpms dip when I let off the gas and turn the steering wheel or put it in gear. I am going to pull them off in the morning and check to clean or replace. I will let you know how that works out.

Matt
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Wednesday, September 22nd, 2010 AT 1:35 AM
Tiny
ROMA0301
  • MEMBER
I pulled off the iac and cleaned it good, looks shiny and close to new. There were some rub marks on the tip of it but didn't look too bad. I put it back in and started the truck. I ran a little better but still rough and back firing. I pulled off the egr and cleaned it. I noticed that one of the nuts that was holding the egr was only finger tight. I put a new gasket on and put it all back together. It runs smoother now and not so doggy when I turn the steering wheel and brake. It is still back firing though. You can hear it when it idles. Dah dah dah boom dah dah dah dah boom boom (my engine sound effects). Not really loud but you can hear it in the idle. When I speed up with the pedal 3/4 of the way down and let off it back fires a couple of times. If I rap the gas hard it also back fires. I don't know if this is the rotor, or the distributor cap, but im thinking it is electrical. Any suggestions?

Thanks again, we are getting there
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Wednesday, September 22nd, 2010 AT 1:22 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Yes, in 1988 they still had a distributor cap, rotor. Ahh the good old days. If you haven't tuned it up in a long time, that may not be a bad idea to check. Also, based on the miles, you may want to check the engine compression. There could be a valve that is allowing pressure to by pass and create a backfire.

I will add that often times, a back fire is created by a vacuum leak. I say that because I want you to keep it in the back of your mind so you are checking while working on other things. One last thing, when it backfires, is it through the intake or exhaust?
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Wednesday, September 22nd, 2010 AT 1:30 PM
Tiny
ROMA0301
  • MEMBER
The back fire is coming from the exhaust. I replaced the cap and rotor and wires and plugs two or three years ago. I am going to pull the cap and the rotor to have a look and it. I will also check with the parts store and do the compression test if they have a tester for my truck. I will send and update later.

Thanks again
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Wednesday, September 22nd, 2010 AT 2:07 PM
Tiny
ROMA0301
  • MEMBER
Ok I did the compression check and this is what I got (148, 150, 135, 138, 140, 150, 130, 148) They all seem to be within the acceptable ranges. Drove it still have loss of power, back firing, missing. I am going to check the rotor and distributor cap, tomorrow morning. I ran out of light tonight.

Thanks
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Wednesday, September 22nd, 2010 AT 8:17 PM
Tiny
ROMA0301
  • MEMBER
I also noticed today that my temperature reading on my instrument cluster is not registering a temp. The rest of the gauges are working fine and the truck is warm, but I don't know the temp. Will I have to replace the entire cluster or just the temp or could there be another reason its not reading the trucks temp?

Thanks

matt
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Thursday, September 23rd, 2010 AT 12:06 AM
Tiny
ROMA0301
  • MEMBER
I got the temp sensor working again. I also checked the distributor cap and rotor. They were used but still looked new. There were no carbon marks inside the cap. I cleaned them up and put it back together. The truck runs better but still has a miss. The idle will run smooth and then fall off, and pick back up to normal idle. If I pop the gas if back fires. It runs worse when its cold, then better when warm but still misses. I am not sure what to do now.

Any suggestions?

Thanks
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Thursday, September 23rd, 2010 AT 7:48 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Chances are the temp sending unit is just bad and not the cluster. As far as the compression, usually you don't want more than a 10% difference between cylenders. You have a couple that exceed that. Have you had a chance to check the other things?
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Thursday, September 23rd, 2010 AT 9:59 PM
Tiny
ROMA0301
  • MEMBER
Well so far I have checked the distributor cap, rotor, plugs, wires, and no problem. I did the compression test. I pulled the EGR and IAC, cleaned both of them. Replaced the oxygen sensor. I checked all the vacuum lines for breaks, nothing. I have not tested the vacuum pressure of the lines or the pressure of the EGR to see if its functioning properly.

This is the update for now, I will try anything you suggest this weekend. Just let me know
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Friday, September 24th, 2010 AT 10:20 AM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Sorry about the delay in getting back to you. I lost my father earlier this week and things have been.

Have you been able to check the last few things?
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Sunday, September 26th, 2010 AT 12:19 AM
Tiny
ROMA0301
  • MEMBER
Sorry to hear about your dad.

This weekend I drove to my friends place to replace my CV shafts. On the way there the service engine soon light came on so I pulled over shut the truck off stuck the paper clip in and got Code 32. I looked in my chilton book and it said egr vacuum loop is bad. I pulled off the line that comes from the MAP sensor and had a friend hold his hand over the end and I reved the engine and he said there was vacuum pressure. So I think my EGR valve is shot.

Would you also think this could be the problem?

Also My power locks don't work and only my drivers door power window works. I tested the motor on the passenger side and it works. But there is no power to the switch. On the drivers door there is power to the passenger window switch but when I plug in the working switch it doesn't roll the window down. So I from what I can tell there is no power to the passenger door at all. And the drivers door I believe is just switches, but I don't know if the bad switches in the divers door would affect the passenger door.
Suggestions?

Thanks again

matt
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Sunday, September 26th, 2010 AT 12:09 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
Matt,
Before replacing the EGR, remove it, clean the carbon from it, check to make sure the valve is working and clean the intake where it attaches of carbon. Also, check the vacuum to it to make sure that hose isn't plugged.

THe electrical issue seems to be coming from the main switch on the driver's door. That is where the power supply comes from.

Let me know what you find.

Joe
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Monday, September 27th, 2010 AT 1:38 PM
Tiny
ROMA0301
  • MEMBER
I pulled off the EGR and cleaned it good, cleaned the opening on the head too. Put it back on and it still stumbles at idle. I pulled the hose that runs from the MAP sensor to the EGR and had someone hold their hand over it while I reved the engine, and they could feel the suction. I think my EGR I bad, but I'm still not 100% sure.

I tried working switches in the sockets. I got the power door locks to work, it seems that problem was the switches on both doors. I checked for power to the plugs and the plug on the drivers door that controls the passenger window has power, but when I put in a working switch the passenger window doesn't work. I checked for power on the passenger door window plug and it has no power but the door lock plug does. I put switches in the drivers door passenger window and the passenger door window and neither work. I tested the passenger door window motor and it works in both directions.

Suggestions?

Thanks again
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Thursday, September 30th, 2010 AT 2:02 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
If you put vacuum to the EGR to see if the valve is working and seating.
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Friday, October 1st, 2010 AT 12:18 PM
Tiny
ROMA0301
  • MEMBER
I pulled the vacuum hose off of the EGR. Plugged it and reved the engine. It still had a small back fire. I put the vacuum hose back on and reved the engine and it back fires when reved. Both ways the has a rough idle. I hook up the vacuum hose and reved the engine while holding the EGR and it didn't move. I pushed on the EGR and it tried to kill the engine. It lost power and almost died both times I pushed on it.

Is there anything else I could try or is this were my problem is?

I tried working switches in the window control sockets. I checked for power to the plugs. The plug on the drivers door that controls the passenger window has power, but when I put in a working switch in it the passenger window doesn't work. I checked for power on the passenger door window plug and it has no power but the door lock plug does. I put switches in the drivers door passenger window plug and the passenger door window plug and neither roll up or down the window. I jumped the passenger door window motor with a battery and it works in both directions.

Suggestions?

Thanks for all the help!
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Thursday, October 7th, 2010 AT 10:46 AM
Tiny
JACOBANDNICKOLAS
  • EXPERT
I think the EGR is playing a role in the problem based on your description. I would remove and clean it to make sure the valve is sealing. Also, make sure there is no carbon build up in the intake as well as the EGR.

Now for the window, it sounds like there is a break in the power supply to that door. I would start checking at the rubber boot area in the door jamb. Chances are there is a break in the wiring there.

Let me know what you find.
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Thursday, October 7th, 2010 AT 9:49 PM
Tiny
ROMA0301
  • MEMBER
Well I cleaned the EGR and the part of the intake were it sits. The truck still runs rough. It runs better if I have the vacuum like that runs to the EGR plugged with screw, but even then it still is rough. When the vacuum like is plugged the idle will be steady then drop then come back to steady idle. When I have the EGR hooked up it almost seems like its missing, and backfires more.

I checked the wiring from the door to the door jam. No cuts or anything. I pulled the boot off and pulled out the wires and found a connector there. I unplugged the connector, and tested it. No power from the connector either. I am thinking it must be closer to a control panel. I don't have a wiring diagram for the window system. The chiltons wiring diagram doesn't have the power window or locks. So I don't know were the power to that window comes from.

Suggestions? Thanks
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Tuesday, October 12th, 2010 AT 4:02 PM

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