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Olds 88 (1998) Low fuel pressure

My 88 “Rosie” has been having problems for months. It hesitates, stalls, and sometimes backfires. This is an intermittent problem, though it is happening more often lately. I took it to local auto shop and their computer showed Rosie was throwing a misfire code as well as a lean code. I took it 3 months ago, and I took it last week, got the same codes. The lean code led me to check the fuel pressure. I did and while it should be healthy 48-55 psi, I got 48 for awhile then 15 PSI! Now I’m thinking fuel delivery. I started with the filter. Sure enough it was dirty. Weeks later, the problem persists. I replaced the fuel regulator (ok, mostly because it was cheaper than the replacing the fuel pump, which I really didn’t want to have to do.) Still, no luck, so I finally caved and spent $200 and 12 hrs dropping the tank to replace the fuel pump. Guess, what? Still no dice. I’m still getting the hesitation and the misfiring. I haven’t put the fuel pressure tester back on it (been busy) so I don’t know if the pressure is still low or if these other things are even related.

I thought my logic was infallible. Let’s face it, there’s a finite number of parts that control fuel pressure. I’m running out of parts to replace. Where did I go wrong?

The only thing I can think of that could still do it is perhaps the fuel pump relay? I dunno. Would it work off and on or fail completely? I’d love to get some opinions on this.

When chasing a problem like this, I always use an old sealed beam halogen headlight to make sure you have good power all the way to the pump. That way you’re putting a load on the circuit. Voltage meters wont tell you if the circuit can carry a load. Of course you could go from the front to the back. I think you can go to the Zones website to look at diagrams and locations of components. Don’t forget to check your fuel pressure to make sure you’re still chasing the right animal.

was the engine running when you did the test ? Did you check the fuel pressure regulator ?

Yes, turned the ignition (but not start) first to let the fuel pump “charge” the lines. The fuel pressure went to 55 then it drops to 48 after starting the engine. Then at some point it also dropped to 15 Psi While Running.

Yes, I replaced the fuel pressure regulator.

You might be thinking along the right path about the relay. One thing I would try is to swap the FP relay with another relay not important to engine operation and see if it helps.

OK. I meant to check if there is vacuum at the fuel pressure regulator. If not you have a blocked vacuum port at the intake, just follow the line and use a can of fuel injector cleaner to clean it. Check the hose for cracks.
There is not too much to say, assuming that the car is in shape, no accidents with body damage near where the wire travel to the fuel pump. The fuel pump is activated by the ecm trough a relay (fuse block behind the passenger kick panel, marked N) that receives power from a 20 amp fuse (#6 on the same block as the relay). You can try to remove the relay and bypass it to check if the pressure stays even, just be careful not to short the ECM.
The attachment is a diagram and some more info. Just be very careful so you do not damage anything else. You could try a new relay, but what about if the problem is in wiring ? you will drive yourself crazy trying to find it. If you see that you can not guess just take it to as mechanic. Sorry to say that.

Since I last posted, I did put the fuel pressure tester back on it. Sure enough, it was struggling at 18 psi. It is also still throwing a P171 code, which is lean bank 1, I believe.

Thank you Pete, I’ll check out the relay.

Castironman, I did check the vacuum hose. It was pulling a vacuum. I also checked to see how the engine responded when I removed the hose. It checked out. (I can’t remember off the top of my head, I suppose it would decrease the fuel pressure). No accidents or body damage, but I’ll definitely take a crack at those diagrams and see if I can find this thing. Thanks so much for the great info!

A guy today told me it could still be the fuel pump because I used the off the shelf one from autozone, not the manufacturers. I (politely) told him to take a hike, probably because I just would hate to admit I spent so much effort doing it wrong. Should I be listening to him? The probability of getting the exact same (somewhat baxar) symptoms 2 days after the brand new fuel pump goes in just seems astronomical to me. Open to thoughts, though, (and I promise I won’t tell you to take a hike). :wink:

Did you replace the fuel pressure pulsation damper when you replaced the fuel pump?

It’s a check valve located on the fuel pump assembly and if it fails it causes fuel pressure to bleed off.

I believe the pulsator was built into the new fuel pump. I’d feel like a doofus if it wasn’t, but I’ll tell you one thing, it would not fit. The new fuel pump was the length of the old fuel pump and the pulsator together. It had a plastic component on the top that the old fuel pump did not have. I had to assume it was the pulsator.


My fuel relay is hot. Not melting, but significantly hotter than it’s neighbors.

Been reading around on other forums, some guys are having similar problems and one symptom in particular caught my eye. They say their power windows don’t work when it happens. Random. But Rosie has been doing this, too. They said they were going to buy the book and find a shared ground for the FP and power windows. Sounds smart to me. It does have all the earmarks of a loose/bad ground connection as far as I can tell. I’m a mechanical guy, though, not so much electrical. Does anyone know where I can find this type of diagram without buying the book? Castironman, I checked the ones you sent and learned a lot from them. I suppose the one that might answer my latest question might be higher level (that is, an overview of more systems and components). You guys rock. Thanks.

Find the return line to the tank from the fuel rail, pinch it off, recheck your fuel pressure after it sits and see if the pressure drops. If it doesn’t drop then the problem is in the tank, more than likely the pulsator.

Ground for fuel pump relay is G101. This ground is attached to the engine next to the oil pressure switch. Oil pressure switch is attached to rear of timing cover. For some reason I can’t find diagram for power windows.

If your fuel pump relay is hot, then either its defective, or the socket it is in is loose. Check the socket and replace the relay and you may fix it.

Peter, thanks, I’ll check that ground out and clean it. I haven’t spotted a diagram for the windows, either. I got my hands on a Chilton, but no mention whatsoever of power windows. Hmmm…

Tardis, I’ll try that, too. I hope I don’t seem tight by asking, is there a way to check the old relay before I buy a new one? It just seems like every time I turn around I’m buying a new part and it doesn’t fix the problem. I’d just like to confirm the old is bad before I chuck it, eh? It’s a 12 V system, so if I put 12V across the coil, I could check the resistance in the arm, maybe? I guess, it could still be bad, because it seems to be an intermittent problem. Hmmm… The Adventure Continues…

You should have a relay in that block that you can trade with it that’s not going to have anything to do with the engine. Swap them around and see what happens.

Ok, I dove back there and I found what looks like a fuel pressure sender. Near it is a tiny black wire with a connector. Following the black wire it disappears deep into the bowels of the engine. I climbed up on top of the engine and reached down in and still could not find the termination.

Question: Would it be safe to jumper from this black wire to the frame and see if it solves it? Then I could dig around to find the termination. I’m more of a mechanical guy, so I’m thinking a ground is a ground. (Begs the question, if it’s not important why did they put it way the heck in there?) Worth a shot, though?

While you had the tank off did you look at the condition of the inside? A rusty tank will quickly plug a new filter. Did you check the sock (if it has one)?

these are the diagrams that I have for your car. If you can reach and pull the relay you can test it but it has to be accessible to a meter. Get a piece of stranded wire, pull the insulation and separate the strands. Wrap the wires around the terminals on the bottom and bring it up to the top along side the relay, make sure it will not touch each other. Put electric tape around the relay and now you have the wires on the top. You can now read the numbers on the bottom and write next to the wires so you will know what side to test. If you have with you you can also tap a test light so you know when the pump is on or off visually and take voltage readings. To really know if the pump is not working well you will need a scope with a low amp probe to see the sine wave off the wires.

more diagrams.

more diagrams.