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HI Folks….I’ve got a 94 Jimmy with the 4.3L Vortec in it. It started idling all over the place a few weeks ago and I haven’t been able to narrow down the problem. It starts a little rough and idles between 700 and 1500 in Park a few seconds after ignition. Once I put it into gear ( anything ) it smoothes out and is actually drivable. I don;t have any problems once moving, but it will hiccup a bit as I sit at a light. I’ve replaced the IAC, it’s got a brand new exg=haust from the manifolds back and I recently cleaned out the EGR and it’s passage. I’m stumped on the association with shifting gear….what effect does this have ?? Seems to run a lot better when it’s cold too !!!!!
Thanks everyone, I look forward to hearing from you.
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What do you mean “association with shifting gears?” Are you referring to it smoothing out when trans is put into a driving gear, or when it is shifting gears? Does it have the remote oil filter near the fan shroud? Have you pulled any codes on it?
Yes, remote oil filter on the fender well….I haven’t pulled any codes, no CEL either. I’m definitely thrown by the “gear change” thing, kinda wierd. When I drive it, it really pulls, sitting at a light, I get a little knock every once in awhile….like a stutter or something. Never stalls though. It’s a true Vortec with the covette style intake right up front, electronic. EGR underneath.
Thanks for replying.Sounds like a vacuum leak although vacuum leak symptoms are usually worse with the engine cold.
Is this a MAP or MAF engine?
I think it is a speed density design (MAP sensor only, no MAF). Let me know.
Also do you have access to a scan tool?
Some fuel trim numbers would be great to know in this situation.
You could have an issue with the fuel psi regulator leaking too but that would be rich conditions, rough idle for sure, but NOT high idle complaints.Yeah, it’s got a MAP only I think. I do get a fairly strong fuel odour once in awhile, wondering if maybe the FPR or Nut Kit is leaking. I don’t have access to a scanner, but I should be able to dignose with a Fuel Pressure Gauge no ??? I’m hesitant to open up the intake plennum and start poking around, might be my only option though at this point. It’s crazy the way it bounces around, sounds like I’ve got a can or something, lol…I know that if I hook up a FP gauge that it should hold around 57-60 KOEO, if you turn off the engine it should remain pressurized for a few seconds and SLOWLY drop off. If you turn the engine off and it plummets down to zero right away it’s a bad Regulator. I guess I could open it up and see if it’s leaking. If either side of the plennum is cleaned out, that’s the side that’s leaking. UGGGGGGH !!!!!
Thanks Danner !!!!Try tapping on the idle control valve while the engine is running. If the idle changes while you do this then I would recommend you replace the IAC and then recheck for the condition. Many people try to clean these but report mixed results.
Brand new IAC, it was the first thing I thought of when it started to act up. Do you know what the pintle is supposed to be set at ??? I think it’s 1 1/2″….you have to repress the spring and screw it in or out to adjust it, wonder if mine isn’t extended all the way. BTW, it’s usually the electronic EGR you have to tap on, lol….any idea if it could be the TPS ??
Thanks Eric.I have had quite a few run in’s with the chevy 4.3 vortec. If you have trouble finding out what is causing these funny symptoms, try looking at the intake manifold. They are notorious for leaking. It will make the vehicle run weird for a long time before it goes completely.
Sounds like I might as well get down to it and open er up, lol….this is by far the weirdest engine configuration yet !!!!!! Still don’t understand the correlation between the idle issue and changing gear, lol. Thanks GM !!!!!
Thanks All !!!!!I had symptoms similar to this on my gf camaro. I had to change the maf,aic,o2,and fuel pressure regulator. I,m noy sure if the plenum gaskets were leaking but I changed them since I had to take it apart to get to the fuel pressure regulator. If you smell alot of fuel try pulling the vacuum line for the fuel pressure regulator off. If there is fuel in the vacuum line your regulator is bad. you will also want to get your hands on a fuel pressure gauge to make sure your fuel pressure is with in spec.
Does anybody know what the CPI replacement part # is, or where I can get one ( for how much ). I do know that it’s a MPFI unit that uses a sequence instead of firing all 6 at once ( what a waste )
UGGGGH !!!! Getting complicated !!!
Thanks again !!!Thanks for the info Bud…in the Vortec the whole upper intake is under pressure, so there isn’t a “line” to the fuel pressure regulator, just an orifice that uses the manifold vac. I guess if wanted to I could pull the upper, power up the pump remotely and watch for fuel spitting out of the orifice, or the nut kit tubing….sounds like fun, lol !!!!! I’m starting to think it’s a terribly bad TPS…I’m hoping.
If you have a scanner, that is capable you can watch the throttle position sensor in real time. You can see if there is any huge drops or increases in the signal.
I apologize for missing that you had already replaced the IAC. That said the more I think about it the more I think you should check for vacuum leaks. These can upset the mixture particularly at idle causing rough running and a fluctuating idle. It’s worth checking out in my opinion. BTW if you don’t want to use carburetor cleaner you can also use water.
UPDATE….I accidently found 3 vacuum leaks at the rear of the intake plenum, lol…they’re all in different places, probably pin head in size. I was doing something else and sprayed some cleaner on my hands, the mist must have gotten sucked up because the engine surged a little, that’s when I took a closer look. I don’t know if it will fix the problem, but I’m replacing the gasket set for the entire plenum. Still don’t understand why it mellows out in gear….anybody ???
I’ll let ya know what happens !!!!!
Thanks again !!I’m back and I found something strange….I found a sensor, or something with a one wire connection on the passenger side of the block right beside the #2 plug wire.
Can anybody name this part ?? I found out that mine is cracked in half, lol.
Thanks !!!!
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