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I recently bought this car and I have a drivability issue I need help with. The car is a 305 5 speed, headers, cats removed, and egr vacuum lines plugged. The fuel pump will go to approximately 40+- psi when I turn the key on, but immediately drops when the relay cut the pump power. The car starts fine and will idle as long as you want. The pump holds about 40+- psi when the car is running and gains a couple pounds when you hard rev the engine, pressure never drops while sitting still. The seller said it has a new TPS (I checked the ohms to verify), IAC, plugs and wires. I have installed a new pressure regulator, cleaned the injectors, air temp sensor under the plenum. The car will crank and idle with no problems, it will rev sitting still with no problems even when warm, It seems to drive normal driving no problems, the issue comes in once there is some heat (around 180 or so and up probably) and at medium to hard acceleration it wants to spit and buck. When I go back to normal driving it runs OK. I assume it must be the fuel pump since it is about the only thing not replaced? Sorry for the long description, but I wanted to put as much info as possible.
Take close up , detailed, in focus pictures of your injectors and any part numbers on them and by where the injector connector is looking for a stylized "RP" logo and post them in your thread. You should be able to rotate the injector some if need be to get the pictures.
If your car has its original GM Rochester Products fuel injectors then they are junk and should be thrown away and replaced with the proper Delphi fuel injectors for your application.
My initial hunch is you are correct, I had similar behavior with my first GTA when the fuel pump was going bad - that being said it wont hurt to verify some other things before dropping tank. How are the condition of your ignition components? cap/rotor/ignition module (get it tested)/pickup coil - these could be corroded and causing issues under heat/load.
I did not have a gauge on it then. I do not have a permanate gauge in the car.
Need to see the pressure when the symptom is happening. When the symptom is happening.
Doesn't need to be a permanent gauge. Duct Tape it (or affix it how ever you prefer) to your windshield or bring it into the car so that you can see it while driving....while the symptom is occurring.
Need to see the pressure when the symptom is happening. When the symptom is happening.
Doesn't need to be a permanent gauge. Duct Tape it (or affix it how ever you prefer) to your windshield or bring it into the car so that you can see it while driving....while the symptom is occurring.
I will put the fuel pressure gauge on it and run it down the road.
Did this problem just occur or has it been this way since you purchased it?
Yes, it has happened since I bought the car, maybe around a month or so. I changed the typical parts hoping it was one of these, other than the fuel pump. It has not had the check engine light on at any time.
Need to see the pressure when the symptom is happening. When the symptom is happening.
Doesn't need to be a permanent gauge. Duct Tape it (or affix it how ever you prefer) to your windshield or bring it into the car so that you can see it while driving....while the symptom is occurring.
I put the fuel pressure gauge on the car yesterday and ran it down the road. The pressure did drop to 25-30 psi when I ran the car up to around 3/4 throttle, but the car did not jump and buck. At normal driving the psi stayed around the 38 -40 psi range. I think some of the difference is the car did not have alot of heat in the engine yesterday when I drove it. It was threatening rain so it was a quick run down the road and back and it never acted up. Maybe the pump is getting hot the longer I drive. Needless to say with the pump dropping pressure, I will be replacing it soon
IMHO I think you need a tune. My best freind has a 92 G92 car, 5 speed. He experiences the EXACT same issue with the bucking @ high rpm/acceleration. His started after an injector change. Usually when mods are made to the car the tune needs to be adjusted. This is the case with your car, headers, not cat, no EGR. This could be enough to throw your fueling off and causing a lean/rich condition in the higher RPM's when the engine is hot which is what causes bucking/hesitation. The computer sees the EGR system so with it just removed you are throwing something off just in that system. With headers, if the O2 sensor wasn't changed to a heated one it will read wrong as it does not come up to temp like in the stock manni's. Are your injectors original?
Before the tune this happened every time he drove the car, now after a round of dialing the tune in it very rarely happens any more. It also helped his overall performance and the car runs better now than it has in a minute!
I put the fuel pressure gauge on the car yesterday and ran it down the road. The pressure did drop to 25-30 psi when I ran the car up to around 3/4 throttle,
Originally Posted by vinny R
IMHO I think you need a tune.
I think he needs a pump. I think that seems pretty obvious at this point. The tune isn't the cause this driveability issue.
Also, he doesn't NEED a tune b/c of exhaust and EGR. Car will run fine...even great, on the stock tune with exhaust and no EGR. Many an F-body owner has ran their F-bods with exhausts, EGR's missing...and a lot more, and had satisfactory results (no driveability issues). Myself included.
A tune would help (if it's any good)...but it isn't a NEED. If course, a tune would help, even on a box-stock car, right? Right. Every car "needs" a tune, but it's NOT HELPFUL when people recommend tunes to "fix" driveability issues that are caused by actual problems. That is called, "Giving bad advice". Fix the problem...then consider a tune for the reasons that you would tune any car.
Last edited by Tom 400 CFI; 09-03-2024 at 09:37 AM.
Every car "needs" a tune, but it's NOT HELPFUL when people recommend tunes to "fix" driveability issues that are caused by actual problems. That is called, "Giving bad advice". Fix the problem...then consider a tune for the reasons that you would tune any car.
Your opinion Tom, all I have to say is it what's fixed my buddies "drive ability" issue which is the same exact issue the OP has.
My opinion? That's years of experience, driving the OP toward an actual diagnosis.
Originally Posted by vinny R
Your opinion Tom, all I have to say is it what's fixed my buddies "drive ability" issue which is the same exact issue the OP has.
Did it though?
Originally Posted by vinny R
My best freind....
.....now after a round of dialing the tune in it very rarely happens any more.
Doesn't sound FIXED, to me. So "get a tune" advice was really, a WAG.
Giving advice to people needing help that is based on "a buddies problem" that was sort of, maybe? fixed, isn't really good advice. I'm sorry to be an a-hole, but it isn't. Giving diagnostic advice to HELP (teach him to fish) the OP identify the problem, that is giving good help. As a mechanic, and the boss of mechanics for 30+ years, that's how we do it. We diagnose first. The OP identified 25 PSI fuel pressure under load. That is an objective problem. That needs to be fixed....then you posted that he should get a tune?
Did you read the thread I linked? "It needs a tune! It needs a tune!". Was that good advice? No, it wasn't.
Last edited by Tom 400 CFI; 09-03-2024 at 01:28 PM.
I put a new fuel pump in over the weekend, took it out for a test drive and got about 1 1/2 miles and the pump stopped. It seemed to have fixed the stumble I had been having before shutting off. I drug it home and could not hear the pump kick on, so I got busy for a few minutes, then went back and laid under the car with a stick and moved the bulk head connector around. I hooked up my pressure gauge to make sure it wasn't pumping and when I turned the key the pump kicked on. I turned the key again to prime the system and then I started the car. I let it idle watching the pressure and it acted like there was no problem. I turned it off and restarted it multiple times (no driving time) revved it several times and no problem. I hope it was a loose connection, but I need to make sure my neighbor is home before taking it down the road again. I did notice my electric fan did not turn on once it got up to temp, so I will be replacing the fan sensor.
Has anyone removed the in tank pump and run an external pump on one of these cars. If so what tank pickup was used. It would make like much easier when the pump dies.
You have three things to check, at this point: the fuel pump relay that ULTM8Z brought up, the fuel pump fuse, and the oil pressure switch. The pump should have kept running on the oil pressure switch, even if the relay were bad. But it didn't, so that already went bad previously...or the fuse blew, which would cut power to both the oil switch and the relay.
Last edited by Tom 400 CFI; 09-04-2024 at 08:31 PM.
how's your fuel pump relay? That could be the problem. In-tank pumps should generally be pretty reliable.
I have not checked the relay with a multi meter yet. I did hear the relay clicking on and off and could feel it also. Initially the pump still didn't kick on, but did later, which is why I think it was either the bulk head plug was maybe loose or the pump stopped for some reason. It has been raining every afternoon when I get home, so I have not had the opportunity to check and see if the car will crank.
You have three things to check, at this point: the fuel pump relay that ULTM8Z brought up, the fuel pump fuse, and the oil pressure switch. The pump should have kept running on the oil pressure switch, even if the relay were bad. But it didn't, so that already went bad previously...or the fuse blew, which would cut power to both the oil switch and the relay.
I know it's not the fuse, because it started working again and I had not changed the fuse. I don't think it is the relay because I was driving the car when the pump stopped working. I could hear the relay clicking when I was on the side of the road, and once I got it home. I have not checked it with a multi meter yet. Maybe the oil pressure switch, but the car had oil pressure just before it shut off, I am very much a gauge watcher in my older cars.
The relay should also be providing power at the same time, in parallel with the relay. The importance of the oil pressure switch in this case is one of two things:
1. If the oil pressure switch is functioning properly, the problem that caused the pump to stop is somewhere in the circuit that is common to BOTH the oil pressure switch AND the Relay. You can test the oil pressure switch, and if it's good, then for sure it wasn't the relay, either. It was something else.
2. Is the oil pressure switch bad? If it is, and then the relay could have caused the problem. Could have. Not likely, but possible.
Anyway...that's an interesting correlation between wiggling your bulkhead connector and the pump working again. I'm can't recall if the FP circuit runs through the bulkhead or not but if it does, I'd scrutinize that.
08-26-2024, 07:44 AM
The OP identified 25 PSI fuel pressure under load. That is an objective problem. That needs to be fixed....then you posted that he should get a tune?
