bad idle speed

Yes, that is a very basic resposne. What I would suggest is this. First, check around the engine for damaged or disconnected vacuum hoses. They are about 1/2" in diameter and provide a route for engine vacuum to travel. If there is a leak, it can cause a low and rough idle. Next, as far as the throttle body, I think what the original post ment was to try a cleaner that you mix with the fuel in the tank. If you stop at a parts store and ask for one, I'm sure they will have a few to recommend. The idle air control valve is a different issue. It is mounted on the throttle body (on top of intake). It's purpose is to maintain a steady idle. When they go bad, usually you will experience a looping idle (up and down).
Let me know what you find with the hoses and if the injector helps.

Joe

Thank you Joe for your help. This is the thing: my GMC Sonoma 91 starts ok, runs fine for a few minutes and several miles, when warm starts the problem: at traffic stops I have to put it in neutral or park and press gas pedal because very low idle speed, sometimes even turns off. I have changed myself lots of parts: tps, pcv and egr valves, put fuel cleaner in tank already,went to the mechanics they charge me almost $300, checked compression, fuel pump, all ok but couldn't fix it, just yesterday to be sure I changed myself injectors, no luck, I am getting desperate don't know what else to do, I am going to check the hoses like you say, more carefully because already did that too, maybe I missed something.The thing is I have taken good care of this truck even changed eng and trans, new paint job plus a lot of parts, but this particular problem is going to make me throw away the truck that otherwise is running fine and still looking good, it's a shame.

Have you checked the EGR? Also, have you checked the intake gasket. In 91, there were problems with the intake gaskets.

I changed EGR myself,I don't know if it's possible the new one is faulty, and yesterday I changed one gasket when I replaced injectors, when I removed fuel press meter/fuel regulator, do you think I have to change all gaskets in tbi unit? Is it possible fuel press meter/regulator make this problem if faulty?

No, the injectors should be fine. When you replaced the EGR, was the intake clean where it mounts?

That is a good point, it's a fact that mechanic work can't be done in a hurry, you have to dedicate time and patience, I have to admit when I installed EGR valve I didn't apply this rule as well as I should, that combined with the fact the egr valve is a little difficult to access, it's possible something is wrong, I will have to remove it again and check if it's clean, but to dedicate the time for this I will have to do it on weekend. I will let you know, thank you very much for your help and your patience with me, unfortunately I can't find this kind of help with the mechanics around this town, and I have tried several shops already it's not uncommon for me to have to finish the work they do to my car after they charge me a lot of money, for example I let them do once transmission maintenance, after that the car was leaking fluid, I went down to check, ALL the screws in the trans. pan were lose, if I didn't check myself I would have ended with a broken trans.

Hay, you can ask anything you want. That is why we are here. I will keep my fingers crossed for you. I know they are a pain to get to and I wouldn't want to have to do it twice. However, it isn't uncommon for the intake to carbon up where they mount. Please let me know what you find. If that isn't the problem, we'll figure it out together.

As far as the shops where you are, they are everywhere. I wouldn't trust many of the ones I work with to change my oil.

Good luck and let me know what you find.

Joe

Hi, Joe it's me again with my old GMC Sonoma, low idle speed problem.I am going to check EGR valve like you told me but I need to ask you a question about something I noticed and is bothering me big time: every time this low idle speed start, at the very same time I see oil pressure going down too, sometimes dangerously close to the red line, I wonder if it's normal that oil press go down because of low idle or God forbid, it's the other way around: idle speed happens because oil pressure problem. Thank you again for your help.

Miguel

THe idle speed will play a big role in the oil pressure. If the idle gets low enough, the pressure will drop. Just make sure the oil is full. I feel confident what you are seeing is expected because of the low idle.

Hi, Joe, I removed EGR valve to check as you told me, it's clean, unless the new EGR valve is defective, the cause for the low idle speed is not this valve. I checked all hoses again, the only thing I found is that months ago while changing some of these old hoses, there is one, is not a flexible hose but a plastic tube L shaped that goes from the manifold to the TBI unit, it broke, couldn't find a replacement, they don't have at the autoparts store, so I had to replace it with a flexible rubber hose, don't know if this can cause a problem, I don't see a difference but again, I am an auto technician, I mention this to give you all the information I can even the small details in hopes that it can be useful to you to help me solve this problem. Thank you very much for your attention. If I was closer to your town I would have driven there and pay you what it's worth to fix the car, I am sure that you would be able to fix it right, because the mechanics around this town, can't trust any of them.

I wish I could be there too. It would be a no charge. As far as the vacuum hose you replaced, as long as there is no kink in it to restrict vacuum, it is fine.

This is a tough one, but we'll get it. The next thing I need you to check is called the idle air control valve (IAC). It is located on the throttle body. What is does is this. When you let off the gas and close the throttle plate, it allows air to enter the engine preventing it from stalling and maintaining a steady idle. Although a problem with this part usually causes a surge in idle speed. However, it can cause a low or high idle.

I have attached pictures of it so you know what you are looking for. It unbolts from the throttle body. At the end is a pintil that moves in and out. That is what allows different amounts of air to enter the engine. When the idle starts giving you trouble, I want you to disconnect the IAC to see if the idle changes. If it doesn't make a change, there is a good chance it isn't doing anything. I would then like you to remove it and make sure it is clean. At that point, have a helper cycle the key from off to the run position. The pintil at the end sould move. If it doesn't, and it is getting power (you can check the wiring harness with a test light), then chances are it is bad.

Try that and let me know what you find. I'm sorry I keep giving you different things to check, but doing this over the internet takes a lot more time. If I was with you, we could do all of this in a matter of 30 minutes.

Take care and let me know how you make out with this.

Joe
PS: I can't remember if I mentioned this, but the early 4.3L V6 engines did have intake gasket issues. I mention it because that may be our next check. At that point, I will explain how to check it. You can check intake vacuum with a gauge, or use starting fluid or carb cleaner to see if there is a change in RPM's when you spray around the intake. However, I would feel better if you had the spray test done by someone with experience. If the spray hits something hot enough, it can catch fire. Not a pleasant experience.

I forgot the pictures.

Joe, I forgot to tell you,when I took the car to the shop couple of weeks ago,they offered to change IAC valve, I could have done that myself and save some money but the car was there, they already checked other things and couldn't fix it so, I let them do it. I am afraid we will have to go to the next step, you say that will be the intake gasket, don't know how difficult this could be, I hope with your help I can do it myself. I am very careful when working on the car so you let me know how is the spraying done and I'll try not to set the car on fire :-) maybe I can do it while the car is still cold and spray a little at a time.

Basically, with the engine running, take carb cleaner and spray all around where the intake bolts to the engine. Also, spray it on vacuum hoses, at the base of the TBI... anywhere there could be a vacuum leak. Please keep a fire extinguisher near by. DO NOT USE WATER TO PUT OUT AN ENGINE FIRE. IT WILL ONLY SPREAD THE FIRE. Make sure the extinguisher is made for oil / grease fires and be careful.

If you hit the leak, one of two things will happen. Either the engine will rev or it will die. You will notice a change in the RPM's.

I will keep my fingers crossed. Let me know what you find.
Joe

Ok, Joe, thank you, I will do it this weekend to dedicate all the time it needs. I have a fire extinguisher in the garage. I'll let you know.

Just be careful and I'll look for your reply.

Hi, Joe, I just used almost a whole can of carb cleaner, sprayed all over, I repeated the process again and again, not one bit of a change in RPM, the engine didn't react in any way to all the spraying. This is getting tougher than I thought. Also, I am thinking, if there was a vacuum leak the car would have the problem since the start, but I notice the low idle speed starts after I drive for about 10, 15 minutes and several miles, I wonder if there is a part in the system that start to fail when the car gets warm.

I have to be honest, I'm not sure where to go next other than the computer itself. I'm going to have a different tech review our conversations and see what he thinks. Maybe a fresh mind will help.

Ok, Thank you very much.

Not a problem. I already copied the URL from here and sent it to him. I have one other person that I plan to send it to. Expect a response from them. Also, keep me in the loop.

hi, joe, pretty much summed it up, the only things I can add is that the timing may be slightly off and that it is possible that the vacuum passage in the intake manifold for the egr valve is really clogged with carbon and needs serious cleaning.. the iac should definitely be cleaned or replaced and the bore that it sits in. the throttle body altogether may need cleaning, and as simple as it sounds, be sure that the airfilter and breather(if equipped is replaced, cracks in the vacuum hoses is a big culprit, and the accordian hose coming from the airfilter box to the throtte body must be looked over, if it has a simple crack or hole or is not connected tightly, your gonna have these problems, these are all possibilites.

Ok, I will keep you informed, I am really determined to fix this darn car even if I buy a new one for several reasons: you know you get attached to your old cars like they were pets :-), also I have gone thru a lot with this truck, moved to this town with it with nothing but thousands of dollars of debt, started a small commercial cleaning business, paid all my debts, even bought a house all with this truck it's the only car I had since 1997. Besides of this another reason I have is that I want to prove a theory I have: american cars, with a few exceptions, have always been great, the problem is this: for what I've seen over the years, people buy foreign cars and treat them like beautiful, delicate ladies, but when they buy american cars, they trash them, abuse them with the idea: because is american it have to take everything I throw at it. When I bought this truck, was 6 years old, was smoking like hell, I didn't want to change the engine right away, so a friend of mine that is mechanic and I opened the engine to do some patch work until I was ready to change it, I tell you this, I had to remove the oil with my hands, was not oil, it was mud, the previous owner never changed the oil. So, I will keep on looking for help even if I have to go to GMC factory and talk with the president of the company :-)

I couldn't agree more. I just sold a 93 dakota with 225K on it. It still ran great, but they stopped making certain parts for it that I needed. Stupid cab mounts (rubber) were no longer available from Chrysler and aftermarket were only available for 2wd. Interesting, I saved that truck from the junk yard years ago. It needed a 2" weld on the frame. Sounds similar to you. I used it for everything. Hell, it plowed everyone's driveway on my street for years. I even used it to build my house. It has been gone since Dec. 22, and I miss it. The new truck isn't the same. When I say new, I mean 13 years old. Don't ever let anyone tell you american cars are junk. I answer around 1700 questions a month. Believe me, they are all junk when they need fixed. I answer as many for imports as I do domestic. The only difference, the domestic are usually much easier to repair. I guess, you can get a bad one no matter what you buy.

You are right. Let me ask you one more question, I received a message, I guess from your friend, among other things we already checked, and obvious ones like changing the air filter, he mentions something about the timing, I didn't know time adjusting can be done in this car, I wonder if this is something I can do myself, if so, maybe you can help me with that. Few months ago the pulley in the power steering pump broke, you can imagine what happened, big mess, the belt was shredding in pieces, still I kept driving a couple of blocks to a parking lot, I wonder if this could cause the timing to go out of adjustment.

The pump issue shouldn't have done anything to the timing. What you can check is ignition timing if you have a timing light. He was most likely thinking that perhaps the timing chain may be stretched (from miles), and could be causing the issue. If you can get your hands on a timing light, let me know. Basically, there is a wire that goes to the #1 spark plug wire, you have to disconnect a wire from the distributor, start the engine, and the light will flash everytime that cylender gets spark. On the harmonic balancer there is a timing mark. On the timing chain cover there is alignment marks which indicate how many degrees before or after top dead center the spark ignites the fuel. From there, you turn the loosten the distributor and turn it until it gets the light flashes exactly when the line on the harmonic balancer touches the timing degree you want it set to. It sounds hard writing it out, but if I was there, we could check it in about 5 minutes.

Ok, it really sounds a little bit complicated,but I am going to check how much it cost me the timing light, if it's reasonable price I'll buy it and I'll let you know to try to do it myself with your guidance, I guess it will be a lot cheaper than take it to the mechanics, besides I don't trust them to do the job right, months ago I let them change the upper control arm, they told me, the car is ready, I told them: no it's not, you didn't do the front end alignment, they noticed I am not a complet idiot, admited it, took the car back to the shop and did the proper alignment, oftentimes I suspect they don't do all the work they are charging me for. So, I will see if it's possible for me to do the timing myself with your help.

Before purchasing one, contact a local parts store and see if they will rent or lend one to you. Most nationally recognized parts store will do it. While there, ask that they show you how to connect it because I won't be there to see what is needed for that specific light. Let me know.

Ok, I'll do that, thank you.