Hello to all,

This is my first post although I have been researching this forum for a while. A little info: I purchased my 1999 blazer about six months ago and its been nothing but trouble. The guy that I bought if from convinced me that it was a "good" vehicle and that the engine had just been swapped. I purchased it although it felts sluggish and slightly missing thinking that it was merely a cap and rotor issue. After changing cap and rotor, map sensor, battery, altenator, plug wires, plugs, transmission solenoid, cleaning MAF sensor; the head gasket gave up and I had the engine replaced with a 100k engine from Keystone. When I picked up the vehicle it was idling rough and had hesitation throughout the throttle range, but my mechanic told me that he could not determine the problem and asked me to drive it for 500 miles and if it didn't get better to bring it back and he would find a solution. It has been steadily running better but still retains it lumpy idle and hesitation. A new issue has arisen; it has what I believe is called roaming idle where the idle wants to remain at 1500 rpm as I am slowing down making it difficult to brake. If I take it out of drive the idle instantly returns to 600 rpm. My two thoughts are EGR and IAC. Not sure, thoughts?

Thanks

 

You should of just replaced the head gaskets on the original motor. the 100 k motor, did they rebuild it first? After 100 k your pretty much waitin for somthing to fail while hoping for the best.

 

The fact that it drops right back down to normal idle after putting the transmission in neutral or park makes me think that somehow the PCM is not giving the IAC the right commands when it is in gear. It is normal for the IAC to open up a bit when putting the transmission into gear to compensate for the added load on the engine.

Have you or your mechanic looked at the IAC counts & other live data on a scan tool to see if there may be any hits as to why it would overreact? Have you cleaned the IAC valve? Maybe it gets stuck at the position for 600RPM out of gear and the overreaction is due to the PCM commanding it out further & further until it unsticks itself...

 

Hard to believe, acting like that, it's not throwing any codes.

I've seen this problem described before but cannot remember what it turned out to be.

I know.. disconnect your cruise control linkage at throttle see if it still does it.

 

The problem with the roaming(I guess that is the correct term)idle just began at lunch. I will not have time to pull it and clean it now due to an obligation but I will attempt tomorrow. Update: On the way home from work at a traffic light in drive, it would surge from 600-900 RPM and back as well as lingering at 1500rpm when I would come off the pedal at speed. I just wanted to make sure that this was more likely a IAC issue and not TPS or EGR.

Thanks for the help

I also forgot to mention that I have been getting misfire code for cylinder 4. I don't necessarily believe that the idle issue and misfire are related, the misfire could in-fact be due to the spider. The spider that I am using came with the replacement engine and at least one of the injectors are likely partially obstructed, my mechanic said that the one that is on the truck is better than the one on my prior engine. If cleaning the IAC and EGR do not resolve the misfire and hesitation, the spider is on the list. That is if my wife doesn't set the truck on fire first.

 

Cleaned EGR last night and didn't experience any change in performance.

I believe I mentioned that the blazer has been driving extremely week and misfiring throughout the throttle range. Before I had the engine replaced, I previously replaced the coil and ICM. My mechanic didn't swap them on the new engine, but gave the whole assembly to me to take with me. In despair, I swapped the whole ICM/Coil assemby at lunch a to my surprise, my blazer has increased in power but a slight hesitation is still in there. My next step will be to re-check plug wires and plugs as well as cap-and-rotor. I am fully convinced that this is an electrical issue, not fuel related.