Engine Performance problem
2000 Other Chevrolet Models V8 Two Wheel Drive Automatic 86800 miles
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I am at a lost on this issue. From a stand still every once in a while (happening much more frequent as time goes on) when I get on the gas lightly or heavy I would get a hesitation sometimes for up to 3 seconds (or more) before the vehicle would take off like it should. Also when I put the vehicle in reverse I've noticed what seems like the idle speed possibly fluxuating (not sure how much but at night when the lights are on you would see the lights dim as you hear the motor "surge") and it appears to be getting worse over time for both issues. In drive it is not nearly as bad and happens here and there. The tranny was just rebuilt just last year, fuel pump replaced about 5 months ago, just replaced fuel filter, plugs, wires, coil, TPS, MAP sensor and exhaust man. on both sides due to both being cracked. I have check fuel pressure and the lowest I saw it when driving was 52 psi but average is 55 psi. I have cleaned the mass air flow sensor (everything looks good from a visual inspection after cleaning) and looked at the electronic egr valve and the valve appears to be moving free. Please help any info would be great. I don't get any codes and don't want to just throw parts at it. The vechile is a Chevy W4500 that has a 350 motor and a 4L80E tranny. It is basically a Isuzu box truck but with Chevy parts and logos etc. Any help would be awesome for I don't want to get in a accident because of a hesitation issue and not being able to get out of the way quickly enough. Oh and by the way the vechile drives just fine once I get past the initial hesitation from a stand still.

First thing is are there any engine modification? Or air intake mods? If not check for vacuum leaks..get a can of spray gumout, spray the vac lines and maniflod area, when the idle changes, youve found the leak! You want the engine running and spray on the outside but do direct the stream onto the hoses, if it is a vacuum leak, the engine idle will change speed, then you have detected a leak, repair the vacuum leak and see how it runs! If that doesnt do it, you may need a new idle air control valve, it's bolted to the throttlebody. If you have one of the oil soaked air filters, change the MAF, cleaning wont work! I have done a few of these for this, and Chevy has a TSB on it.

Everything is stock on the vechile. I never had a hesitation issue until after I got my transmission rebuilt / replaced. After the transmission was taken care of I remember getting a very slight hesitation but then it went away for almost a year until it came back ten fold about a month ago. I've taken the vechile to two transmisison shops and they both experianced the problem but say that they don't beleive it is a trany issue. There has been no mods to the vechile. There really isn't any vac. hoses (a couple) on this thing but I will try that gumout stuff and update you on that. Yeah I was actually looking at replacing the idle air control valve. Can this cause hesitation from when I'm at a stand still in drive and then getting on the gas? For I don't have any hestiation issues after I'm actually "moving" and idle's just fine if it's not in drive or reverse for then it kind of surges. The air filter is a stock paper type filter that was also just changed.

Pull the IAC off and see if it;s loaded up with carbon, if it is I replace them, cleaning does'nt usually work! And use some throttlebody cleaner on it!

First off thank you for your help. I can't tell you how much this means to me. I pulled off the IAC and it didn't look bad but I decided to replace it. After replacing it I drove the vechile down the road for about 5 miles or so around 40mph so that the computer can re-learn and when I got back home it appears that the idleing issue has possibly been resolved. For when I first started the vechile it appears the the idle was really low sometimes around 420 rpm and now it is around 620 to 600 rpm average when in drive and reverse. I don't know if the hesitation is resolved yet but I want to drive it for a day to find out for sure so I will update you on this. However when I did get home I checked for DTC's and now I've noticed that my HTR light is flashing on my scan tool which wasn't before. I don't have any pending codes or any codes at all right now but can you tell me what the HTR light means for I can't find anything explaining this. The scan tool I have is a Innova 3130 (www.canobd2.com) from Schucks (the best one you can get there from what I've seen) which can read live data and is the tool that I've been gathering information from during this whole proccess. Again thank you for your help and support!

since you just replaced a monitored part, it may mean that the system, even though the PCM has no codes, it could be that it's not ready for en inspection of the emmisions system, but I dont really know, I don't have that scan tool.

Here is an update on the issue. I still have hesitation but it is about half as bad and more so on when I go from a stop to go with a lighter throttle. Since replacing the IAC valve the vechile has died a total of three times so I think the idle may be a little too low at times. I have noticed that every once in a while I hear the idle kind of surge but then goes away. I have tried to figure out how I can check the vacuum on this vechile but I can't find any place on the intake man. to test for vacuum. I have included some pictures of the motor.

Question... Do you think that the IAC vavle that I just replaced is possibly bad? Or do you think that I may have something else that may also be bad? I'm looking into getting a car repair manual for this vechile looks as if they arn't cheap.

I was just doing some more testing and when using my scan tool I've noticed that the MAF sensor is reading when in nutural around 600 rpm about .9 - 1.1 lb/min. Now when I put it in drive every once in awhile I get some surging from around 500 rpm to 600 rpm and the reading on the MAF sensor shows higher around 1.5-2.1 lb/min. Is this right? I looked at the STFT and when this was happening it was around -1.5% to 0. Does any of this make sense? Sorry to ask so many questions.