1993 gmc 4.3l v-6 service engine soon light came on

recently my service engine soon light comes on when i drive the truck and starts running rough.only when i turn the truck off and let it set for about ten minuites and restart it it goes off and comes back on.i put the gm code reader on and it read code 35 idle air control system problem, can not set desired rpm. replaced (iac) valve cost 53.00 disconnected neg. battery cable to reset computer for 30 seconds. light still comes on and flashes code 35 don’t know what to do. i feel like i have wasted 53.00. please need help!

I saw your comment on the “What would you like to see on cartalk” thread. First you probably did waste $53.00. Code 35 indicates that the ECM can not achieve the proper engine rpm. This can be caused by a faulty IAC but not usually. The most likely cause for this code is a vacuum leak. The first thing you need to do is check the condition of all the vacuum lines under the hood. Replace any that are broken. If they all look ok post back and we will try something else.
Also in the future if you post a question and get no response return to your post and reply to it. This will bring the post back to the top of the list. I am sorry to say some posts get overlooked do to the large volume of questions. Hope it helps.

Thank you so much for your response dartman69 . I will try that and post the news soon. Again thank you

This is the problem most people run into… they get the code and replace the part. In most cases this doesn’t fix the problem.

There is a diagnostic chart that goes with the code 35. It gives you step by step instructions on what to check and then what component to replace. This is where you need to start.

checked all vacuum lines they are all in good shape.what next?

where is this chart at?

Can you indicate the approx. engine rpm when the check engine light comes on?

yes it jumps from 700rpms to 200rpms real rough.

Are you saying it drops from 700 rpm to 200 rpm. Or is it jumping from 700 rpm to 2000 rpm?

back and forth 700rpms to 200rpms jumping

Yeah, where is that chart? I would like to get a copy too! LOL. I guess you get one of these when you graduate from a automotive repair school. Being a good tech isn’t as easy as it looks.

I guess you could consider the money you spent as a lesson learned in car repairing. You can’t jump to conclusions by just getting a code number since other factors can effect the outcome or results.

I too would say this problem may be due to a vacuum leak or possibly a fuel delivery problem. Did you check for a intake manifold leak or have you noticed some loss of coolant possibly leaking into the intake?

Had the intake manifold gasket replaced in september 07

Use a can of propane with a control nozzle attached. Idle the engine and play the propane around the intake manifold to cylinder head areas. If the engine rpm increases, or the engine runs differently, you’ve found a place which is losing vacuum.

Done this but no change in RPM! What to do?