Question : 3100 - Losses power under light load

[thor124]

Hi, I have a 1995 Monte Carlo LS, 3100. Recently while crusing along doing around 80KPH (engine at 1600rpm or so), if I give it a litte gas the engine kind of stutters and looses power. If i step on the gas it down shifts and the problem goes away. It also does the same thing if accelerating hard from a stop.

The car recently had its head gasket replaced(both heads and the LIM were gone. Go GM!!) about 7500KM ago, fluids look good, tranny fluid changed at the same time as the head gasket(some fancy synthetic junk the mechanic suggested).

Reading through some of the previous posts here, I've come up with a few thoughts on what the problem might be.
1) Vacum leak -- how would I check this?
2) ICM
3) Tranny problem - Really hope its not this.

Anyone have a suggestion?

[gsEnterpries]

Is the check engine light on? If it is that is the first place to start. When was the last time you had a tune up done? There are a couple of things that this could be.

[fastback4]

That's odd. My 1997 3100 with 15" tires runs at 2,200 RPM's at 80 MPH.
The most popular suggestions i've read for diagnosing this problem are.........
1. Engine light on?
2. Have car checked for codes.
3. Fuel pump filter dirty.
4. TPS bad.
5. Faulty or stuck EGR valve.
6. Bad FPRegulator

[thor124]

That's odd. My 1997 3100 with 15" tires runs at 2,200 RPM's at 80 MPH.
The most popular suggestions i've read for diagnosing this problem are.........
1. Engine light on?
2. Have car checked for codes.
3. Fuel pump filter dirty.
4. TPS bad.
5. Faulty or stuck EGR valve.
6. Bad FPRegulator

Ok, first off 80KPH(roughly 50MPH)

1. no SES
2. no codes
3. new fuel filter didn't resolve(was my first thought)
4. gotta check
5. will check
6. whats a fpregulator?

Found a vacum leak at the PCV valve(cracked "boot"), fixed, helped the rough idle. running a bottle of injector cleaner through it too.

[IsaacHayes]

Fuel Pressure Regulator. Check the pressure with a gauge on the end of the fuel rail and see what the pressures are at idle with and without the vacuum on it. Also make sure it's not leaking fuel into the vacuum line.

How are the plugs? Did you pull the front ones and look at them? Those might need replacing.

Also a bad/worn out O2 sensor won't throw a code (unless it fails). When they get old they will read lean and the computer will throw extra fuel at it. It will cost you MPG and also make it bog off the line loosing power and responsiveness. This may be your problem. They say they should be replaced every 30,000 miles. My car ran so much stronger with a new one.

How are your plug wires? Check the resistance on those too.

Are you getting any knock/pinging? Maybe you got some bad gas. I know my car used to ping in really hot weather on 87 octane and that causes it to pull timing and then it was a dog until it down shifted. Now they put 10% ethanol in all the gas here and that raised the octane to 89 and that is enough to keep it from pinging. (I ran 89 non-ethanol before they did that and it helped, got better mpg too due to keeping full timing). Not sure why it pinged, pistons look clean on top, but oh well. Since then I've also modified my intake manifolds so I don't even know if it would ping any more in hot weather on straight 87 octane (can't get it here any more).

[thor124]

Thanks for the suggestions guys. I'll pull the front plugs to have a look.... They were new(under 5000km) before the head gasket went but i guess all the antifreeze everywhere could have fouled them. Then onto the EGR(cleaned this once about 2 years ago), and TPS. I don't have a pressure gauge to measure the fuel pressure, if the above doesn't solve it, I guess i'll bite the bullet and have it into the mechanic for a tuning.

[IsaacHayes]

Make sure the EGR pintels are not stuck open. I had that happen on mine and was wondering why the engine would want to die and not start and run rough... Pulled the EGR for another reason and look what I found? lol.

O2 very well could be your issue. Or plug wires. TPS can be checked with a multi meter.

[thor124]

I wouldn't be surpised if it was the O2 sensor, it has been running rich for a while now. I'll let you guys know!

[IsaacHayes]

Put a new one on there then!! Don't get Bosch, get AC Delco or Denso.

www.sparkplugs.com has Denso's for cheap, and they are good sensors.

[thor124]

ALDL

OK a little update...

I finaly had some time to spend on the car over the weekend. Had a look at the front plugs, they were ok. all the plug wires were ok, coils passed the resistance test.

I'm suspecting the MAP sensor at this stage, but of course w/out a proper scan tool trying to watch its operation is a real pain in the rear.(Chilton manual tells me to "back probe it with a voltmeter"... but there is no openings in the connector to do so)

To that end i'm looking at purchasing a ALDL cable, as for verifying the function of the various emission sensors, this seems easiest. Not being terribly familar with the ALDL, if there are any codes stored I will be able to read them correct? Also monitor in real time various sensors?(keep in mind I have one of the pain in the rear OBD1.5 cars...)