Chevy 3100 V6- sag/hesitation.

[Stevien1]

I was hoping someone here had some insight on fixing a GM 3100 V6. The car is a 2002 Chevrolet Malibu with 86,000 miles. The car had a rough idle and wouldn't get out of it's own way- meaning that there is almost NO acceleration at WOT. I know it's not a TM, but its down right dangerous in traffic (being so slow to take off).

I replaced the spark plugs and wires (both where original) and the idle is nice and smooth now! However, its still hesitating on accel- part or full throttle. If you take off from a stop and floor it, it'll slowly rev up to 5000rpm and just sit there and not shift or continue to accelerate. In neutral it revs up to 5500rpm and hits the rev limiter.

I pulled the fuel filter and it seemed to be restricted (compared to blowing through the new fuel filter). So I replaced the fuel filter, it's a little better, but still dangerously slow. I checked fuel pressure and it's 50psi at idle, 60psi with the regulator vac line off. (book says 52-59psi). I went for a drive with the gauge still hooked up and it read 60psi when it started to hesitate.

No DTC's are set, the TPS reads smoothly when applying throttle, engine has 18inHg vac at idle, 15inHg in drive. Running out of ideas and time- anyone have a "been there, done that" or something for me? Thanks in advance!

[turbovanman²]

Plugged cat?

Is there any codes?

Tranny could be stuck in 2nd

Clean the MAF sensor. Take it out, use a small pick or screwdriver and remove the screen, it comes out quite easily. Get a toothbrush, some carb cleaner, brake cleaner and compressed air. Spray the dirty side with carb cleaner, take the toothbrush and very, very, very carefully clean the little wires. Reclean with carb cleaner and blow out with air. Now reclean with brake cleaner and blow again. Reinstall but don't put the mesh screen back on.

[Stevien1]

Plugged cat?

Is there any codes?

Tranny could be stuck in 2nd

Revving it up in neutral with the vac gauge connected doesn't show any problems, I'll go drive it with the vac gauge on. I tapped on the bottom of the cat and it doesn't sound like anything is loose, but I know that's not the end-all of tests. ;) Its not setting any codes.

Trans shifts fine at part throttle, but at WOT it's really slooooow to get up past 3500rpm. It'll climb up to about 5000rpm and just sit there untill you let off. Also at maybe 1/4 to 1/2 throttle it'll sag on acceleration and not go anywhere. It doesn't "buck" and feel like it's misfiring- it "feels" like either the cat is plugged or the fuel pump isn't flowing enough. I've been reading around the net and it seems fuel pumps don't last too long on the '97-'03 Malibu's. I'm wondering if running around with the restricted fuel filter wore out the fuel pump? It's hums along a bit louder than it used to.


Clean the MAF sensor.

I'll do that and report back, thanks Simon! When it comes to anything besides DCX vehicles (especially driveability stuff), I'm lost! :(

[Stevien1]

Update:

Cleaned the MAF, left the screen out- no change. Went for another drive with the vaccum gauge hooked up: 1/4 throttle in 2nd gear, 0 vac when it started to hesitate. Stopped and shifted in to manual 1st- 1/2 throttle all the way up to 5000rpm, had approx 5inHg the whole way. At WOT, the vac gauge stayed at zero (no positive pressure). Crusing along at 40mph in 4th with very light throttle- 10inHg.

The upper intake isn't smoking hot (like if the EGR was stuck open), no codes at all. It idles great (for a 3100) and revs up fine in neutral. I don't know, I'm thinking about throwing a fuel pump at it- but that's a $200 guess. :(

[turbovanman²]

Well ususally the fuel pump will give you a massive hesitation but most cases, they just die.

I would say drop your dp and go for a drive, it really sounds like your cat is plugged.

[Vigo]

just leave the gauge hooked up to it while you drive (fuel pressure). if it cant supply enough volume under load then it wont make enough pressure either.

[Stevien1]

I'll give that a shot then. Have to wait until morning, it's a little late to be flying down the street with open manifold. ;)

Thanks Simon.

[Stevien1]

just leave the gauge hooked up to it while you drive (fuel pressure). if it cant supply enough volume under load then it wont make enough pressure either.

I did drive with the fuel pressure gauge hooked up. It read 55-60psi when it was hesitating. I wish I could find something wrong or broken! :(

[turbovanman²]

Well I know these engines inside and out so based on what you've told us, I would be shocked if it wasn't the cat.

[Stevien1]

Well I know these engines inside and out so based on what you've told us, I would be shocked if it wasn't the cat.

I'm hoping it's plugged up- it would be nice to find something wrong for a change! I'll post back tomorrow after I drop the exhaust and go for a ride.

[Vigo]

it sounds like the cat to me too but the fuel test was a prereq to dropping the exhaust in my diagnostic flow. :P

[Stevien1]

Another update:

This morning I seperated the exhaust from the manifold and went for a drive...... MUCH BETTER!!!!!! :D It actually spun the tires for a second when I mashed from a slow roll! WOT runs out to 5500rpm and shifts into 2nd like it should, part throttle no longer hesitates and it'll actually accelerate.

Unfortunately for me, no one in the metro Detroit area has a direct-fit cat converter in stock. :censored: So I have to wait untill tomorrow morning for one to come in on order. Hopefully this will take care of things for a while. I'll post a conclusion after I swap in the new cat.

Thanks guys for the help!

[turbovanman²]

No problem, glad we could help.

[SQUIRRELMASTER]

Make sure tha car is not California emissions. This can cause issues with a direct fit cat...!

[boosted]

Check with your local dealer...It may still be under GM warranty..In Canada the warranty for a catalytic convertor is something like 139,000km and 7 years..I'm not sure what the States are for warranty emiision related stuff

Chris

[SQUIRRELMASTER]

86K will not be under warranty... It is mileage specific not year...

[Stevien1]

Federal converter warrenty is 8yr/80,000 miles- bummer.

I installed the new cat this morning and it's running good (for a 3100 that is!) It has more "pep" now, so hopefully it won't get ran over in traffic! :) Thanks again.

[boosted]

Always seems like the cars break just after warranty..Sorry about your luck
but :thumb: you got the car running good again. I'm a GM tech here in Canada and sometimes its amazing what hidden warranties are out there beyond the regular 3 yr 60,000km. GM has in place a powertrain warranty for
specific vehicles in certain years..up to 100,000km and 5 years.

Chris