1997 GP GT Running extremely rich and pressure build up in the fuel tank

Hi guys, It's been a long time!

I'm having this weird problem with my car, 90% of the time it will start to run rich, until it reaches normal temp, to the point that I can smell it coming out from the exhaust. LTFT is at around -15% and it seems to go down after cruising for some time but will go back to negative at idle. My first suspect was the FPR so I checked it and found no gas smell on the vacuum hose, then I decided to run the engine with the vacuum hose disconnected and see if any fuel would come out, 5 minutes later and it was still dry.

There is a chance that the MAF sensor is bad, I'm 90% sure it is, because it will sometimes read 2.9-3.1 gps at idle and according to the rule of thumb of 1gps per liter of displacement, it shouldn't go that low but I might be wrong about it; I disconnected the MAF and the smell was still there, today while driving it was reading 5.45gps (A/C was on) and the smell was still present; car also feels sluggish from time to time only at normal operating temperature (which I assume is because of it running rich).

There is also a vibration felt through the steering wheel only while idling in gear at all temps.

Today when I got home and after I turned off the engine I heard a noise like water was running. It was coming from the car so I tried to find where the noise was coming from and it's from the fuel tank. I removed the fuel cap and lots of air came out (or sucked in?). I reinstalled the cap and pressure started to build up almost immediately! I have a video, which I'm sharing, so you can see what I'm talking about; I was even able to hear the fuel tank expanding because of the pressure inside, not sure if you can hear it towards the end of the video (when I put the phone under the car).

http://youtu.be/jeDwiBwbRaQ

It also happened today that I was cruising and the engine hit the rev limiter after I stepped on the gas (WOT) to overtake another car. I immediately removed the foot from the pedal and gave it less gas and it acted normal, no slipping from the transmission at any other time.

There is one more thing that I'm not sure about it. Every morning, the engine will sputter for 1 or 2 seconds after starting it but it will be fine for the rest of the day. I once tried disconnecting different sensors from the engine and the only one that seemed to make a difference (it didn't sputter) was the TPS.

Things done so far:

- I replaced the EVAP purge solenoid (P0441 was present) back in february.
- Rubber pieces at charcoal canister also replaced, old ones were rotted to the point my hands would look black just by rubbing against the hoses.
- Upper intake manifold was also replaced by me in november last year.
- New NGK spark plugs and OEM spark plug wires last december.
- Cleaned the MAF sensor with MAF cleaner, no dice. Used brake clean, no change. It's a reman Cardone from autozone, which tells us it's 90% sure it's broken.
- New Duralast MAP sensor back in 2010 because a mechanic broke the original.
- 10qts transmission fluid and filter replacement 2 weeks ago.
- Fuel pump was also replaced back in 2009 maybe, can't remember the exact year. The old one would start making some kind of a hissing noise after prolonged driving.

NO DTC'S PRESENT. I once checked Mode $06 data with a mid-level scanner I have (AutoXray EZ-scan 6000) and saw a test had failed, I think it said something about the MAF and EGR. I'll have to check it and post it here.

Haven't connected my fuel pressure gauge tester. Since I can smell it running rich, I didn't think it would be necessary.

I think that's all I can think of right now, not sure if I'm missing something but I'll make sure to post anything else.

I'm thinking that there could be a problem with the EVAP system that's causing the excessive pressure in the tank but I'm not sure what that could be. Is there any chance that the pressure is also causing the rich mixture?

I'll appreciate any help you can provide.

Oscar.

Well, if I ever run into you, I will recognize you by your hand ,wrist, and forearm....."Hello, hand!":grinno:

I am surprised that an EVAP code is not set for that high pressure......it certainly looks like either the tube to the vent solenoid is plugged or the vent solenoid is stuck in the closed position....with key off, there is no power to the solenoid, so it should be open....

As for running rich, if air filter is ok, and there is no obstruction of air into the throttle body, the usual culprits are the f/p reg, injectors, MAF and O2 sensor....

If you have a scan tool to read O2 data, if the O2 reads rich, disconnect the MAF and see if the O2 sensor readings become normal....if they do, I am thinking MAF.....

Never had much luck with aftermarket MAF's........OEM's, either new or reman, are more expensive, but when it comes to any kind of electronics that go in the car, I always go with OEM.....

What are those NKG's doing your vehicle? You get OEM wres, and then go and put those plugs in?

Thank you for your input Tech II


I am surprised that an EVAP code is not set for that high pressure......it certainly looks like either the tube to the vent solenoid is plugged or the vent solenoid is stuck in the closed position....with key off, there is no power to the solenoid, so it should be open....


I'm surprised too about no codes being set. Today I took some time to inspect the charcoal canister and related hoses; to my surprise there is no vent valve on the system! I thought I was going nuts and even took a look under the access cover for the fuel pump but nothing... So I decided to have another look at the manual and it turns out there is no such thing on this car.

Anyway, while checking the canister, I removed the cover on top of it and it turns out that's the actual vent for the system. As soon as I popped the cover I saw lots of dirt on it so I decided to blow some compressed air to clean it

The canister:
https://www.dropbox.com/s/vnd0bpv99jkxfda/IMG-20140423-00891.jpg

With the top cover removed:
https://www.dropbox.com/s/qy32x1k6zvrp79x/IMG-20140423-00890.jpg

This is after I got rid of the dirt on the openings
https://www.dropbox.com/s/1pfga1dnsbhewm1/IMG-20140423-00892.jpg

And these are the hoses that I had replaced before:
https://www.dropbox.com/s/4rd8btxosp8tuj3/IMG-20140423-00889.jpg

After the test drive, there was no longer that noise I heard yesterday; I didn't remove the fuel cap.


As for running rich, if air filter is ok, and there is no obstruction of air into the throttle body, the usual culprits are the f/p reg, injectors, MAF and O2 sensor....

If you have a scan tool to read O2 data, if the O2 reads rich, disconnect the MAF and see if the O2 sensor readings become normal....if they do, I am thinking MAF.....

Never had much luck with aftermarket MAF's........OEM's, either new or reman, are more expensive, but when it comes to any kind of electronics that go in the car, I always go with OEM.....


I removed and took a look at the FPR and the filtering screen on the underside looked clogged. I soaked it in some cleaner and I was able to see trough it again, some deposits were left on the cleaner. As soon as I started the engine there was no sputtering nor shaking felt through the steering while at idle in gear. I thought it was fixed but unfortunately the shaking came back after a few miles; I'm thinking of disconnecting the IAC and disconnect one injector at a time as a cylinder balance test, not sure if it could help or what to expect from it, looking at the RPMs drop should be enough, right?

During the small test drive, I watched both O2 sensors and fuel trims. The upstream O2 sensor keeps cycling at all times and goes rich (.95v) at WOT, downstream remains steady at .45v. The lowest value for LTFT was -12% which seemed to happen until I got inside the garage at home, other than that it was between 0 and 7%.

I had already tried disconnecting the MAF and saw no change but I could give it another try.

Getting an OEM MAF would require me to go to the dealer but they are crazy with their prices. I once asked how much would it cost a reman rack and pinion and they wanted $1259 just for the part! I decided to never go back there unless it is strictly necessary; I would probably have to order it from the US, or do you think getting one from the junkyard would be a good idea?.


What are those NKG's doing your vehicle? You get OEM wires, and then go and put those plugs in?

Well, I've mostly used NGKs and autolites, never had a problem. The previous wires were aftermarket (I got the same brand for the 3 cars in my signature) but they al failed after 1 (Town Car),3 (Windstar) and 4 years (Pontiac). Since I think it's too soon for them to fail, I decided to go with OEMs, I found a local place that sells them and there was not much difference in price.

I can get Champion, NGK or Autolite locally, which one of those 3 be better from your experience? ACDelco's are crazy expensive (from my point of view) at $10 a piece but if you really think I'll see better results, I could bite the bullet and give them a try.

Well, if I ever run into you, I will recognize you by your hand ,wrist, and forearm....."Hello, hand!":grinno:

Hahaha, that was funny. Check my picture so you can recognize me not just by "hand", that's my regular outfit...

this is OBD II......there has to be a vent solenoid.....if I remember right, it's behind the splash shield in the left rear quarter(near gas fill tube)....

Hmmm, I'll take another look. The hoses on the canister run straight to the fuel tank and to the purge solenoid respectively, no hose runs to the fender well area.

The manual states it as being in the same location you mentioned, but the "Theory and operation" chapter mentions nothing about it being present in the system or being commanded closed during purge.

Oscar.

In addition to my previous post, I wanted to also post a screen cap with the text from the manual so you can see what I'm talking about.

Oscar.

After all these days of driving the car, the rich smell hasn't reappeared, the excessive pressure in the fuel tank seems to be gone as well.

I checked again at the wheel well looking for the vent solenoid and there is none that I could find.

The only remaining problem is the negative fuel trims. What I've been able to notice is that it will be normal until I get home. The only things I can think of are either a downhill street or the change in temperature inside the garage. Since I was working, I've been using my cheap scanner to check it but today I'll take the other one with slightly more capabilities.

I wonder if you have any idea on why the engine hit the rev limiter the other day, when I first posted, while stepping on the gas to overtake another car. As I said before it never happened before nor it has happened again.

Thanks,

Oscar.

I guess I spoke too soon...

The noise coming from the fuel tank is gone, I opened the cap and there was very little pressure inside so I'm guessing that's fixed.

Regarding the rich fuel mixture, it's still present and it happened during stop-and-go driving (as opposed to my previous reasoning).

- Long term fuel trim was at -22.7%
- MAF at idle was at 2.92 grams per second
- O2 read .895V for a few seconds at the LTFT from above, then it started cycling again.
- Car felt sluggish while the problem was present.

The only difference I can think of between today and the past days, is the ambient temperature; today it was a hot day (around 90 degrees), whereas the past days the temps were lower. Not sure if that could mean something.

I'll appreciate any suggestions,

Oscar.