I've been taking apart & putting back together my 1997 Monte Carlo LS 3.1L all winter but it's running rough and I'm at my wit's trying to figure out what the problem could be!

It's rough starting, cranks and fires for a bit but finally starts with the pedal down a little.

At 2500 rpm sometimes the engine oscillates between 2200 and 2800 rpm. When you let off the gas the engine drops down to about 500 rpm, almost stalls, then goes up to 1200 rpm before settling at 1000 rpm idle. When you hit the gas pedal there's some hesitation and the odd backfire. My fuel pressure at the pump is about 65 lbs, and at the rail is 55 lbs.

The pressure holds for an hour when the engine is shut off.

I think it's fuel-related but I'm stuck! Anyone have any suggestions or similar experiences? Thanks!

 

I literally yesterday had a similar problem that turned out to be a mix between my MAf and IAC...

 

I had this same issue with a 96 Monte that was my sons car and it was a Vacuum leak the Hose that runs from the manifold to the fire wall near the drivers side of the engine. It is about 1/2 inch hose that fell off. Reconnected it and everything was good.

 

Thanks to both of you for the suggestions, I really appreciate the help! Today I cleaned the MAF sensor and the IAC...but no change. I turned up the idle to 1200 rpm so at least I'm not stalling.

In the midst of all this, the car died and wouldn't start...when I unplugged the intake temperature sensor, it started. I cleaned that up and I'm back to where I was before...

I wasn't able to find any detectable vacuum leaks. Motor hesitates, backfires or almost dies if you press the accelerator suddenly. Lots of motor surging at 2000-3000 rpm.

The problem started after I replaced a broken cam shaft which only lasted a year...I'm open to suggestions of what parts to look closer at that could cause these problems as maybe some component is defective but I hate to buy new parts if I don't need them.

The check engine light is on, so maybe there's a clue there? A neighbor up the street has a code reader and should be home this weekend so I should be able to use that to check it out.

 

Could it be a clogged cat?

And autozone or similar stores usually will scan your car for free..

 

Monty, I had a similar problem a couple of years ago with my '95. It also had what felt like some mild transmission "bucking", esp. when going uphill. I too cleaned the MAF and the IAC, to no effect. Tried a couple of transmission additives too, no effect there either.

Aside from a case of the usual bad intake gaskets, the real culprit was the oxygen sensor. When that (and the gaskets) got replaced, all the bad idle, stalling, and bucking problems pretty well vanished.

 

Thanks again for the replies!

No free OBDII or store scan in my area unfortunately...I'm in a small town in the middle of nowhere.

I've noticed some transmission shifting and bucking problems after I fiddled with the IAC...thought I may have done it to myself when I was trying to bring up the idle to 1200 just to avoid stalling.

There are two O2 sensors, upstream and downstream of the cat-conv...which one did you end up replacing? If they're the same part # I'm thinking of swapping them to see if anything changes. Hard to get at the one in back of the motor though...they look a lot like the one on my old Lumina and I should have a spare somewhere from that car although I bet the connector will be different.

I think I might have more than one problem going on...going to see about borrowing my neighbors code reader when he's home and hopefully I’ll get some answers from that!

 

The 1995 Montes all had only one oxygen sensor, and that would be the one behind the engine (by the firewall). I believe that's because they were part of the final year of OBD-I, so all 1996 and newer Montes were on OBD-II and thus got the two oxygen sensors and all the other little changes and refinements.

Your neighbor's code reader should help shed some light on where the problem is, that'll be good to find out.

 

Thanks for the info! Haven't gotten my hands on the code reader yet but I'll post what it comes up with when I do

The Monte quit again when I was out yesterday and wouldn't start...I pulled the plug on the air intake temp and it started and it managed to limp home. Trying that sooner probably could've saved me at least two AMA tows!

Not sure how the car will function with a new IAT but I'm hoping it's the problem, going out to do some more testing now...

 

Definitely check the engine codes when you get a chance... If you order a code reader online they're fairly cheap.