Question : '95 3100 Runs smooth, performs poorly

[Lukeonia1]

So, I have a '95 Grand Prix that I'm planning to sell. It's in good shape with "only" 120,000 miles on it. I got it on the cheap because it needed a new transmission, but the seller gave me a spare so I just dropped it in myself, and it's been a good car since.

Trouble is, compared to my other '95 Grand Prix, there is a very noticeable lack of power with this one. The engine idles fast (around 1100-1200 RPM compared to 800 RPM), it gets poor mileage (14-16 MPG in town, barely 20 MPG on the highway, versus 18/26 MPG), and you have to give it a LOT of gas to get the car to accelerate. Possibly related, it also takes FOREVER to warm up in the wintertime.

Other than these issues, it starts right up, runs smooth and quiet, and has no other problems that I can find. I am planning to sell it so I don't want to put much money into it, but I'm sure I could get more for it if it wasn't so sluggish on the road.

What could be the matter with it?

[Penglii]

vacuum leak?

[robertisaar]

that high of idle can only be caused by so many things....

vacuum leak, malfunctioning IAC, misadjusted throttle minimum stop screw, malfunctioning TPS/PCM, malfunctioning coolant temp sensor.

that's the list that comes to mind. i'd start there and you might fix the other issues as well.

[TGP37]

Any codes to report? That is a good place to start.

[Lukeonia1]

Any codes to report? That is a good place to start.

Haven't run it for codes, partly because I'm not getting a Check Engine light, and partly because the car has no plates so I really oughtn't be driving it.

How about a bad O2 sensor? Acting on a hunch, I unplugged it, and it didn't change the engine's performance at all. Then I went and unplugged the O2 sensor on my other (identical) Grand Prix, and sure enough... the idle was elevated by about 300 RPM above normal, acceleration took about 500 RPM more than usual, and the other symptom I forgot to mention: unusually strong-smelling exhaust. I plugged it back in and everything returned to normal. Interesting to note, in neither car did unplugging the O2 sensor produce a Check Engine light.

I'll see about replacing that sensor tomorrow... it'll be a most frabjous day if that is indeed all that's wrong with it.

[TGP37]

Could be, definately. Be sure the engine was warmed up when you try the unplug test. Because the PCM doesn't use the O2 sensor when cold. The coolant temp needs to cross a setting to allow closed loop operation (using O2 sensor feedback).

I think the o2 needs to fail more then once to trip an SES code, not sure though.

Some cars, if the tune is good, unplugging the O2 won't change much at all.

Some codes are not going to illuminate the light at all, but they are still tripped.



Did you attach the vacuum line to the transmission? Some trans need a vac referrence. If your old trans (4t60e) had a vac line for atf pressure and the new trans did not (4t65), you might have a vac line hanging open somewhere.

I suggest unpluging and blocking with finger the vac port near the alternator and the other up front to the right. Reason I say the vac port by the alt is they run to a vac chamber and I found the plastic check valve seems to break easily when old (95 is getting old). It can be a PITA vac leak to find and can easily be damaged during a trans swap if you didn't pull the entire block/trans out to swap.

You did pull the trans from under?

[Lukeonia1]

Could be, definately. Be sure the engine was warmed up when you try the unplug test. Because the PCM doesn't use the O2 sensor when cold. The coolant temp needs to cross a setting to allow closed loop operation (using O2 sensor feedback).

I think the o2 needs to fail more then once to trip an SES code, not sure though.

Some cars, if the tune is good, unplugging the O2 won't change much at all.

Hmm, I was testing it cold, but it made an immediate difference on the other car, whereas on this one it did not. I swapped out the oxygen sensor anyway, since the old one looked, well, really old. It improved the idle by about 100 RPM when cold, and 200 RPM when hot. It's still awfully sluggish on the acceleration though.

Some codes are not going to illuminate the light at all, but they are still tripped.

Well that's unhelpful of them.

Did you attach the vacuum line to the transmission? Some trans need a vac referrence. If your old trans (4t60e) had a vac line for atf pressure and the new trans did not (4t65), you might have a vac line hanging open somewhere.

I suggest unpluging and blocking with finger the vac port near the alternator and the other up front to the right. Reason I say the vac port by the alt is they run to a vac chamber and I found the plastic check valve seems to break easily when old (95 is getting old). It can be a PITA vac leak to find and can easily be damaged during a trans swap if you didn't pull the entire block/trans out to swap.

Yep, the transmission vacuum line is connected. I already tried disconnecting and stopping up the accessory vacuum line, but it didn't make a difference. I also tried the trick of spraying carb cleaner around the fuel injector ports to see if one of them might be leaking, but didn't notice anything. There's no audible hissing coming from the engine either.

You did pull the trans from under?

No, I raised the engine and transmission out the top as a unit, swapped the transmissions on the ground, and reinstalled them together.

Meditating on it a bit, I DID recall that the car gets an SES light at highway speed. I was told once it was an EGR code, probably caused by a constricted EGR passage. Could I have the opposite problem, an EGR stuck partway open? That seems like it would cause a rich mixture at a given MAP sensor reading, and a code when the EGR fails to open properly. (What is the trigger for an EGR code, anyway?)

[TGP37]

EGR code trigger, good question.


Look around and see if you didn't secure the exhaust feed from underneath properly, causing a huge vac leak...or if the wires are bad. If the EGR is dirty, clean it some...if all else fails and it seems dead or just to clogged up, try blocking the EGR with quarters or a proper sized piece of sheet metal and bolt the EGR on top. A little ghetto but it will prevent the leaking if your EGR is failing open or stuck. If it improves, replace the EGR.

EGR is open at idle and low load and should shut in high load....I don't know the specifics. A failing EGR stuck open can definately reduce power and create the very symptoms you are having...good smooth idle and low load but lack of top end power.

[robertisaar]

with the digital EGR, the solenoids are tested when the vehicle is at a decent speed, very little to no throttle and the engine speed is high enough.

then the PCM tests them by commanding them on one by one to watch their effect on MAP. large enough change when the solenoid opens and the solenoid is considered functional.

EGR open at idle.... i would expect both richness and a rough idle.

[TGP37]

Yeah see, I knew I wasn't 100% on the operation. I yanked mine out. Though I thought they were cracked open at idle to keep nox down?

Maybe the guy has a clogged cat and the back pressure is blasting through a dirty EGR compounding and already iffy condition.

[robertisaar]

the linear and analog EGR setups, maybe, but not with digital EGR. under a certain RPM, they're all commanded off. 1200 seems to be the magic number GM used.