vmatt350

First, I have done quite a bit of reading on this topic and now Im confused.

The car: 93 z28 m6, mostly stock. Fuel pump replaced about 4 years ago with hardly any mileage put on during that time. The opti was replaced about 10 years ago with no further problems (plus it doesnt feel like the opti). Injectors replaced about 6 years ago with SVO's

The problem started about a week ago. It only gives trouble after driving for about 20min. I checked the fuel PSI and its a bit low at 39 psi with the key on. At first prime the psi goes to about 42psi. After about 10min the psi drops about 12psi to 27psi. The car starts fine. The problem will start off very slightly then quickly get worse, almost to the point of undrivability.

Now the problem: After driving for about 20min the car stars to stumble like its running out of gas, mostly at mid to high RPMs. The car quickly starts doing the same thing at lower and lower RPMs until it is also doing it at idle. It will idle rough at about 500 rpms. The car has not shut off on me yet.

Also of note, the first time it happened I floored the gas pedal. The car bucked and back fired then took off like a bat out of hell. The bucking and back firing caused the check engine light to come on but it has not been on since that day about a week ago.

I have replaced the fuel filter and no change.

How long are the codes kept in the ecm?

Thanks for any help. I know there have been a lot of posts like this but none of the ones I found describe my exact problem.

93zstartedme

27 psi is quite low from what I've read. Was the replacement pump stock? Perhaps if it didn't get driven much in the past 4 years the pump whacked out due to sitting most of the time. It may get weak as it runs and heat up. Oh, two other things that come to mind are- the mesh sock in the tank (maybe you got some bad fuel and as it's being drawn through it gets clogged up, reducing your fuel pressure) and- fuel pressure regulator (I don't know about that one though, I think it usually just causes a rich condition). Pull the vacuum line and see if the regulator side is wet with fuel. Hope maybe this could help.

Fatdog2

First I would replace the coil wire.Then,I would pull up to a driveway,and let the car idle for a minute or so,and (at night) see if the headlights flicker.Then ,it could be your coil.If the lights don't flicker,then the manifold intake sensor could be bad.It's that little black thing on top of the engine manifold near the front.You can get a code reader at wallmart(used to be $20.00)or go to shoeboxes site for an easy code reading useing a paper clip.If you don't have a code book,post the code here,or e-mail me for the code the car is sending.Hope this helps.My02

Injuneer

Appears you've only checked the fuel pressure with key on/engine off. Doesn't prove much. Measure the pressure at idle with the vacuum comp line off (41-47psi), then reconnect the vacuum line and the pressure should drop proportional to engine vacuum (up to 8psi drop with a stock cam, less with a more aggressive cam). Then tape the pressure gauge to the outside of the windshield, take it out on the road, and up to max RPM/max load. The pressure should not drop below 40psi.

That will tell you if its a fuel pump issue.

He does not need a WalMart "code reader" with a 93 (courtesy of Shoebox):

http://shbox.com/1/dlc2.jpg

He doesn't need a "code book" either:

http://shbox.com/1/Dtcs.htm

93 codes are on the far right side of the OBD-I page.

vmatt350

Thanks guys, I will try to trouble shoot as you suggested, starting with the fuel psi check while running. I guess I could drive around my neighborhood until the problem begins and check the fuel psi then.

Fatdog2-I will check the coil since in the past the car would not start and the last time that problem happened it was at night and the coil body was arching to the bracket that hold it to the engine. Only happened 3 times and hasnt happened in a while.

Thanks again and I will post when I get the results of trouble shooting.

Oh yeah, the car needs gas, do you think it would be a good idea to put a bottle of fuel system cleaner and fill 'er up or just put few gallons in for now?

bw_hunter

Put in a half bottle of Techron and 5 gallons of gas. Techron is easily the best cleaner I've ever used and the Corvette community sweras by it for their fuel system issues on the C5.....

vmatt350

OK, I pulled the vac line and the fuel psi increased from about 34-35 psi to about 42 psi. I didnt see any fuel in the vac line from the regulator. I am about to go for a ride to see the psi under WOT. Will post results upon return.

vmatt350

Went for a ride (@20min) until the problem began. I watched the fuel psi and it was no different after the sputtering began. I did notice the needle on the fuel psi gage began to jump quickly but it was not constant through the problem and it never fluctuated more then 2psi.

During the drive the psi was about 32 while coasting, 38 under normal acceleration and about 42 at WOT.

So what do you guys think?

vmatt350

Well, I changed the coil (in bad shape anyway) and that didnt help. I took it for another drive and again after about 20 min it started but this time it got worse faster. By the time I got back in the driveway the car was idling at about 500rpm (usually about 800-900). The fuel psi was dancing and dipping a little bit but not by more then 2-4 psi. While revving it in the driveway it would pop and sputter. I just dont get it.

Also, after the car has run for a little while with this problem I noticed the coolant sounded like it was boiling. The pump was recently changed and I don’t have any over heating issues while driving.

Does ANYONE know what this could be?????

***NEW RELATED QUESTION***

So I was bleeding the fuel psi gage and the fuel started spitting out of the hose. It was like the fuel was boiling or something. The gage showed no PSI but "air" kept spitting out. I put the key back on (to prime the system) and there was no psi. It took about 3 times to prime the system before it showed normal psi on the gage. WTF!?!?!?!

Any helpful input would be great!

Hal Fisher

I don't know why every one is suggesting the pump when it looks more like a fuel filter problem? I mean that's what clogs do, the filter can handle low speed but won't let enough to pass for high revs or for an extended period of even low speed.

Hal

vmatt350

Hal-I changed the filter about a weeks ago. The problem was there before I changed it too.

Hal Fisher

OK because the filter certainly seems like the most possible cause. Other than that coils do get hot and stop working and you tried that as well but you said the pressure was dropping greatly. Last thing in line and hardest to change is the fuel pump. Not like that wasn't a problem before. Maybe check to make sure it has a good ground and power first.

Even a very small psi, like .2, will cause the gas to spit out. It takes about 3 times to get all rail and everything filled back up so that's normal.

The coolant shouldn't be boiling because it's under pressure which makes the boil temp much higher. Your gauge was also well below that.

Hal

vmatt350

Could the fuel pump be getting so hot that its boiling the fuel?

Injuneer

I responed to his post regarding the fuel pressure, because he had tested the pressure and didn't do everything that needed to be done. I wasn't "suggesting the fuel pump".... I was just explaining the correct way to test the pressure. Apparently you find this incorrect.

The problem with losing pressure on pump shutoff does not have anything to do with how the engine runs once it starts up. Dropping pressure may make it hard to start, but once the engine is running, a weak fuel pump check valve will have no affect on how the engine runs. He's already checked the vacuum line, and it doesn't appear there's any fuel in it.

That's not "air" spitting out of the fuel pressure test port, its fuel vapor. Now you may have found something. Perhaps the pump is on its way out, and overheating. That may heat the fuel in the tank after 20 minutes of operation, and may lead to vaporization in the pump suction line. That is one possible explanation of the fluctuating fuel pressure when the stumbling starts, but not the only one.

I don't see anything in his results that suggest its the fuel filter - he said he had changed it in the first post. All of his results are perfectly normal until the engine starts to stumble. The question is does the fuel pressure fluctuation cause it to stumble, or does the stumble cause the fuel pressure fluctuation? When the engine starts stumbling, another possible explanation for the twitchy fuel pressure may be that when its misfiring, the intake manifold vacuum starts to jump and dip, explaining the minor fluctuations of the fuel pressure readings.

Have you considered the possibility of heat soak affecting the IC Module? ...or possiibly the optical module in the Opti?

Have you checked the codes yet? They will stay "stored" for a specific number of "drive cycles where the problem does not happen again. Best thing to do, rather than worry about how long the code stays stored is to flash the SES lights to determine if the stored code is still there.

bw_hunter

I really don't think so. However, as Injuneer suggests, heating the fuel near the vaporization point could cause the sputtering you see in the drain line. If that's the case, I'd expect you'd have a pressurization of the fuel tank. Try running the engine with the fuel cap off and see if the problem is different. There is probably a clue in that..

Building on Injuneers comments, maybe you're experiencing an old carb problem....vapor lock. The fuel lines in an F-body run pretty close to the exhaust system, especially if you have headers. The exhaust could heat the fuel in the line, locally raising the fuel temp to the point of vaporization. This causes air bubbles in the line, starving the carb. This problem was supposedly solved with the advent of fuel systems with return lines and FI engines move a lot more fuel than they use, theoretically keeping the fuel cooler....