Thread: No power under load.

Hi all , '02 Grand Prix SE, 3.1 . seems to have no power under a load or under accelleration. Idles great and starts right up. I replaced MAF, new ICM,EGR,muffler,air/fuel filters. Cat conv seems to be flowin good, less than 3lbs back pres on exhaust. Fuel pres never drops below 50 lbs. even under driving condition and when its acting all crappy. car has jus over 109k miles, lookin at the sparkplugs , looks lean, white on plugs as well as inside the cat, looked slightly white like it was runnin lean. not sure where to go next, anyone have any ideas??? btw, not the trans. thx again

no codes present , that what really has me scratchin my head.

I was also thinkin the lean condition might be cause from an intake leak, but it doesnt seem to be idling high and O2 sensor seem to be readin fine. air:fuel is readin about 14:3.1 and timing at idle about 18*, im clueless at this point, jus think im overlookin something

How did this thread play out, is the original poster still around? Again, VERY similar problems to what I am dealing with now, wish people would close out their threads better....

Well, the post was never closed because car is still sitting, engine problem still unresolved, lol. was hoping someone might have some insite to this problem but i guess not. tried a bunch of different things, swapped out a bunch of parts. still scratchin my head. talked to someone on a tech line , they were convinced it was a bad cat.conv. I disagree, even with the exhaust TOTALLY unbolted from the manifolds (zero back presure) still no power.

What was the TPS voltage under throttle? If the TPS is sending a wonky reading that might do it. Also, what were the O2 readings? Maybe the O2 is carbon fouled reading too rich and causing it to pull fuel. IDK really, just didn't see where these were checked.

Not convinced that you can expect normal performance with NO exhaust system downstream of the exhaust manifold. Where will the fuel trims go with the O2 sensors hanging in ambient air? I just came though my problem , it was in fact a partially clogged cat. Car was running as bad as you described yours does. Mine runs like a raped ape now! The odd thing about my case was that I never did get any cat converter codes like the typical P0420 low cat efficiency code, as things got really bad I was on occasion tripping the P0404 EGR performance error code , which from what I have read, is commonly tripped by excessive backpressure from a clogged cat which causes the EGR valve to pop out of the shut position. Another symptom I had was rich exhaust smell getting into the car from the ventilation blower sucking in fumes from the engine bay that were pissing past the exhaust manifold seals on the head exhaust ports due to the excessive backpressure.

You said you were seeing 3 psi of exhaust backpressure. Was that measured at the upstream O2 sensor exhaust port? At normal idle, or a high speed idle? Seems pretty high, shouldn't see any significant pressure at that point at normal low idle. What do your fuel trims look like with the cat installed under load going up a long upgrade? If the cat is clogged you will likely see a bias toward lean (negative number) trim due to a rich mixture. If you are fuel starved ( like if your fuel pump is on its way out) , you would be seeing a bias toward rich trim. I'm mentioning that because my fuel pump actually just died the other day, but I dont think it was related to the clogged cat condition. That said, even with my 3.1's lousy performance with the partially clogged cat, the lean trim bias I was seeing under load was not that gross. But it was there. Another thing I was seeing from the clogged cat was a lot of coolant temp rise under load during uphill pulls, never had that under normal operation and that is now gone with the new cat.

Clogged cats on these cars we enough of a problem that GM extended the warranty on the cats for clogging to 120k miles/10 years. Mine only had 95K but was 13 years old so I was SOL for a warranty replacement from GM. The Walker cat my mechanic put on was about 250 bucks over the counter and I have seem them for about 180 at the usual Internet parts outlets.

Originally Posted by AJ1G

Not convinced that you can expect normal performance with NO exhaust system downstream of the exhaust manifold. Where will the fuel trims go with the O2 sensors hanging in ambient air? I just came though my problem , it was in fact a partially clogged cat. Car was running as bad as you described yours does. Mine runs like a raped ape now! The odd thing about my case was that I never did get any cat converter codes like the typical P0420 low cat efficiency code, as things got really bad I was on occasion tripping the P0404 EGR performance error code , which from what I have read, is commonly tripped by excessive backpressure from a clogged cat which causes the EGR valve to pop out of the shut position. Another symptom I had was rich exhaust smell getting into the car from the ventilation blower sucking in fumes from the engine bay that were pissing past the exhaust manifold seals on the head exhaust ports due to the excessive backpressure.

You said you were seeing 3 psi of exhaust backpressure. Was that measured at the upstream O2 sensor exhaust port? At normal idle, or a high speed idle? Seems pretty high, shouldn't see any significant pressure at that point at normal low idle. What do your fuel trims look like with the cat installed under load going up a long upgrade? If the cat is clogged you will likely see a bias toward lean (negative number) trim due to a rich mixture. If you are fuel starved ( like if your fuel pump is on its way out) , you would be seeing a bias toward rich trim. I'm mentioning that because my fuel pump actually just died the other day, but I dont think it was related to the clogged cat condition. That said, even with my 3.1's lousy performance with the partially clogged cat, the lean trim bias I was seeing under load was not that gross. But it was there. Another thing I was seeing from the clogged cat was a lot of coolant temp rise under load during uphill pulls, never had that under normal operation and that is now gone with the new cat.

Clogged cats on these cars we enough of a problem that GM extended the warranty on the cats for clogging to 120k miles/10 years. Mine only had 95K but was 13 years old so I was SOL for a warranty replacement from GM. The Walker cat my mechanic put on was about 250 bucks over the counter and I have seem them for about 180 at the usual Internet parts outlets.

Well, since the car has been sitting over a year now, i cant tell you exactly what all the fuel trims were reading. Having it hooked up to my scanner, the only code it ever threw was a P0403. All the other readings at the time from what i could tell were in acceptible range. TPS swepped fine 4.5-5 volt WOT... temp sensors seems fine, nothing really out of the ordinary. Back pressure from the exhaust was measured from the down stream O2sensor
at idle and under a load, even with the exhaust completely unbolted it acted no different then when it was hooked up, reguardless of what the ambeint reading where giving it.

I work in the automotive industry, in a small , but very busy Tranmission shop. I mostly install but do some building... still learning. BUT every other shop I have talked to says they would have done and checked all the same things I have already done. So , I pretty much ready to start from scratch all over on it with some fresh prespective. Obviously , it is something simple I am just over looking.

To be honest, the Cat conv. the only thing I have not changed. Things i have changed ...EGR, MAF, ICM, Fuel pump, Complete intake manifold gasket set, ( someone mentioned, may have internal intake leak...idk, but i changed alot in the past for coolant leaks. couldnt hurt), new muffler and even changed pcm and re-flashed by dealer. recent tune-up plugs and wires, even swapped coil packs, no diff.

Anyways, i will try to get back on it this weekend with my Solus scanner and I can let you know what the O2's are reading, but if I remember correctly, think all the O2's were doin thier job.

Thanks, this is the most in-put i have got back on thins car in months, lol , much appreciated

If it is idling and not missing under load, I would doubt you have any significant intake manifold vacuum leaks. One thing bothers me is that you say you mentioned measuring 3 psi of exhaust backpressure at the DOWNSTREAM O2 sensor port. Any backpressure there should be minimal, what you are looking at is the backpressure caused by the exhaust pipe resonator (if any) and muffler, NOT the cat. If you were seeing backpressure there it would have to be from a clog in the muffler, or resonator. If the cat failed a lot of it may have gone down the pipe and clogged things up further down the exhaust. Where did you disconnect the exhaust, directly at the manifolds or on the downstream side of the cat? If the cat has remained in the flow path and is clogged, you will not see any real change in performance. Try measuring the backpressure at the upstream O2 sensor bung ahead of the cat to determine if the backpressure is from the cat itself. And try running the system with the cat installed and the upstream O2 sensor removed, if there is a clog in the cat, the open O2 sensor bung will relieve a lot of the backpressure and you will likely see an improvement in performance. Have you taken the cat off and inspected it for damage/banged it to see if there is loose chunks of catalyst rattling around in there? Considering all you have done the cat seems to me the one most likely thing that could be your problem that has not been corrected.

Took my car on a road trip today mostly highway miles, for about 200 miles. Ran really strong, like a new car, and my gas mileage is back to normal - got 22 mpg, best ever previous was 24 in the summer, and I was pushing it pretty hard today. Coolant temp stayed right around 185, no more big increases after pulling up hills. Runnin down the dream...me and Del were singin!