04 GA SE 3400 chronic P0300/chugging

[J Wikoff]

The wife's Grand Am (3.4L V6) has been chugging and throwing a P0300 for a couple months now.

I've ruled out:
MAF, TPS, coils, plugs, wires, fuel filter, and vacuum leaks.

I've run with the O2 unplugged to stay in open loop, and the problem persists.

It chugs sometimes at highway speeds (a few chugs at a time, every few minutes) and during acceleration (3 or 4 good chugs/predetonations/misses as the revs climb).

I would test the fuel pressure, but there is a power steering line in the way of the test port on the fuel rail. I would replace the the FPR but I think I'd have to remove the TB to get access to it and I don't have a new gasket.

Any thoughts?

[GRRRR8]

Most common issues are fuel pump or cat converter

[CanadianEh]

^ what he said... fuel pump is the first thing that popped in my head

[GRRRR8]

3400 very rarely have fuel press reg issues, but you can disconnect the vac line and see if fuel is in it and then if there is its bad. A mad fuel pump will result in lean misfires (P0300). A bad cat can cause the same symptoms as the exhaust is being forced back through the manifolds forcing the EGR open which causes a lean misfire also. If you have the means to look a parameters, make sure the EGR says 0% at idle, if it gets stuck open it is lean misfires as well as a surging idle.

[J Wikoff]

It idles fine, and there isn't any fuel in the vacuum line. Is the pump in the tank?

I can look at most parameters. I've got the scantool.net program. It's a little limited, though. For instance, it can't see KR. I'll have to check again for EGR, but if it's commanding 0 EGR, but it's open anyway, won't it still say 0%?

[GRRRR8]

It idles fine, and there isn't any fuel in the vacuum line. Is the pump in the tank?

I can look at most parameters. I've got the scantool.net program. It's a little limited, though. For instance, it can't see KR. I'll have to check again for EGR, but if it's commanding 0 EGR, but it's open anyway, won't it still say 0%?

No. Its a linear type EGR that has 3 position sensors so it will know if its stuck. If it idles fine then I doubt its the EGR. If the fuel trims are over + 10% thats another indication of a lean condition, especially if they are 20+.

[J Wikoff]

Hmm. The trims are within a few percent of 0. They were -6.5% before I replaced the MAF with a known good one, now they are about -2%.

[GRRRR8]

Hmm. The trims are within a few percent of 0. They were -6.5% before I replaced the MAF with a known good one, now they are about -2%.

+/- 5 is fine. Other then the code how does it run going down the road and under acceleration hot/cold?

[J Wikoff]

Other than the chugging described above, it drives fine. Hot or cold doesn't seem to make a difference.

[Blackrider]

Check the cat, the 3400's have problems with clogging them. The one in my Grand Am was so bad it I had a hard time reving over 3500RPM and getting over 90KPH. It started with chugging.

[J Wikoff]

How do you check it?

[-Ray-]

An exhaust shop can check the pressure in the exhaust system.

[GRRRR8]

To check it properly you need a backpressure gauge. You can use a vac gauge and under steady throttle pressure the vac should drop to near 0 if its the cat.

[Andy@Livernois]

well... It could be the most prevalent issue on 3400s... Hows your coolant level? Usually when I get a p0300 code its because the intake gaskets have failed and its starting to eat coolant... If so, replace them with the new metal type gaskets...

[GRRRR8]

He didnt state coolant loss or overheat. Another issue related to your post Andy is that if a 3400 has an intake leak and gets hot, go ahead and replace the head gaskets and inspect heads. Since they can only be machined .010. I have found in 90% that when the intake is torqued it lifts the heads and very soon after it blows a gasket.

[Andy@Livernois]

He didnt state coolant loss or overheat. Another issue related to your post Andy is that if a 3400 has an intake leak and gets hot, go ahead and replace the head gaskets and inspect heads. Since they can only be machined .010. I have found in 90% that when the intake is torqued it lifts the heads and very soon after it blows a gasket.

true, but none of my 3400's ever overheated cause the leak was so slow... unless of course I ignored it.

[J Wikoff]

It used to sip some coolant, but I rectified that with a new water pump. I guess it used to weep ever so slightly till one day it was more of a sob and I could find it. But it hasn't used a drop of coolant since.

Would a backpressure guage likely be available for loan/rent at a store like Autozone/O'Reillys/Advance/Napa?

[GRRRR8]

It used to sip some coolant, but I rectified that with a new water pump. I guess it used to weep ever so slightly till one day it was more of a sob and I could find it. But it hasn't used a drop of coolant since.

Would a backpressure guage likely be available for loan/rent at a store like Autozone/O'Reillys/Advance/Napa?

Maybe...or an exhaust shop that you trust can do it.

[J Wikoff]

I don't suppose anyone has an exhaust pressure tester they can overnight me? lol I can't find one locally to borrow. But Napa would be more than happy to sell me one for $220.

[GRRRR8]

You can use the MAP sensor also. If the MAP readings are real high this is an indication of lost vacuum indicating a converter issue. Dont forget to call me when you get a minute.

[J Wikoff]

I removed the O2 sensor and let the car idle. The "intake manifold pressure" didn't change.

*note: this reading is 28.6 inHg with the key on, engine not running. 10 to 11 inHg idling, around 7 if I pull the vacuum line off the FPR.

[GRRRR8]

Check the upper plenum gasket. While the engine is cool, start it up and take some brake clean and spray around the upper plenum gasket and see if the idle changes. 10in vac is very low. It could still have a cat issue as the O2 wasnt enough to let it breathe, but the vac should have changed. Do you have a vac gauge? Just a Sears cheapo will do.

[djchubbs]

to test the cat check the temp pre and post cat i believe there should only be a 20% drop in temp. if the temp post is hotter than pre plugged cat:poof:

[J Wikoff]

Ok, I managed to "adjust" the power steering line enough to get a fuel pressure tester on. 50 psi at idle, 58 if I pull the vac line off the FPR.

I have no way to test the before/after cat temp.

I have tried spraying TB cleaner all around. Nothing changed.

Could this be an EGR issue? Are they cleanable?

I can get a vac guage tomorrow.

[GRRRR8]

Ok, I managed to "adjust" the power steering line enough to get a fuel pressure tester on. 50 psi at idle, 58 if I pull the vac line off the FPR.

I have no way to test the before/after cat temp.

I have tried spraying TB cleaner all around. Nothing changed.

Fuel pressure is good. See if it drops under load (meaning low volume)

[djchubbs]

temp gun parts store 15 20 bucks

[GRRRR8]

Ok, I managed to "adjust" the power steering line enough to get a fuel pressure tester on. 50 psi at idle, 58 if I pull the vac line off the FPR.

I have no way to test the before/after cat temp.

I have tried spraying TB cleaner all around. Nothing changed.

Could this be an EGR issue? Are they cleanable?

You can remove it and press the valve on the bottom and see if its stuck or not moving freely.

[J Wikoff]

Fuel pressure is good. See if it drops under load (meaning low volume)

I can rev the engine, but not take it for a drive with this guage.

EDIT: Holding the TB open enough, maybe 1500-2000 rpm, the pressure hangs around 50 psi. I can hear it missing/chugging a bit.

[GRRRR8]

I can rev the engine, but not take it for a drive with this guage.

Can you get in the car and footbrake it. This will simulate a sufficient load. hold it at 1300 for about 4-5 seconds and see if it drops.

[J Wikoff]

Mother fucker. I went to take the fuel pressure guage off, and the port is spinning with the connector on the tester. Fuck me.

[GRRRR8]

I have seen this happen. Can you get a pair of needle nose pliers to hold the spinning fitting.

[J Wikoff]

If I had a pair that was only 2mm thick...

Is there a fix for this? If I crimp the end of the rail tube down and JB weld it or something? I don't really like that idea, though.

[Chewy]

If I had a pair that was only 2mm thick...

Is there a fix for this? If I crimp the end of the rail tube down and JB weld it or something? I don't really like that idea, though.

I've ground tools down on occasion to do things like this. Just don't use the Craftsman, Mac, Snap-On tools... lol

I hate when $hit like that happens.

Chris

[J Wikoff]

I got a new rail from the salvage yard. Now I just gotta put it in, and then continue to figure out this problem.

[J Wikoff]

After poking, prodding, probing, and pouting... a spark plug wire finally reached out and shocked me. I've tugged on them while running to see if they were loose and if it made any effect and never got a shock. Until this evening when I got it back together after putting in the new fuel rail. I could touch it anywhere in the stretch over the UIM and get shocked.

:woohoo: but still :poof: for being something stupid.

[bigtreepu]

Glad to hear that the puzzle is finally solved! It may have been something stupid, but at least it was a cheap fix! :)
~Matt

[GRRRR8]

Good to hear! I was under the impression they had been changed already.

[J Wikoff]

They were... I replaced them recently as preventative maintenance. But the problem didn't show up for weeks after. So other than grabbing, tugging, and doing a quick spray bottle test while idling and not finding any issues, I figured they were ok. I figured wrong. I also figured, if it was one wire, it wouldn't be a "Random multiple misfire". I overestimated the PCM's ability to sense a misfire on a specific cylinder. I'd pull the rest and test them out, but the terminals tend to pull off because they are so tight and I only have 2 more good spares.