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Weird problem that can't be diagnosed

Hi all,

After going to two different mechanics and not finding an answer, I come to you all…

Car is a 1993 Buick Roadmaster with a 350 V8. Two weeks ago, my car started a thing where I turn the ignition in my car, the fuel gauge goes ALL the way past Full (not normal) and cranks, but won’t start. I had just filled my tank up and was between full and 3/4 when this started.

First time it happened, had the car towed to the mechanic. Suddenly, once at the mechanic, the gauge starts working again and the car starts!

This happened two more times that week, same symptoms. The next week, no problems at all.

On Sunday, I filled up the car. On Tuesday, the car was back up to its old tricks. Took it to another mechanic - tested the fuel pump and said it was fine.

So…what is happening here? The car continues to do this, and it’s only doing it in the morning after the car has been sitting overnight. The only common things I can fine is that when the problems started this week and two weeks prior, I had just filled up the tank to full. I’m thinking that something is loose, like the wiring harness or fuel float inside the tank, or perhaps the sending unit. My searching on the web has turned up nothing.

The fuel gauge thing is weird. Even if it stopped working, why wouldn’t the car crank but not start - there’s plenty of fuel in the tank. I’m completely lost and frustrated at this point. Any pointers or direction?

First thing would be to spend an hour or two under the hood and refresh all of the common ground connections. A fuel gauge indicates a ground problem and if the ground also affects the fuel pump or the computer, you aren’t going to have fuel pressure or the computer firing up the fuel pump. Also when it does it, listen for the fuel pump activating for a few seconds or not to help diagnose. Might be the harness going to the tank also.

In addition to Bing’s suggestions, see if you have access to the fuel pump under the rear seat or the trunk floor. I agree with him that the pegged fuel gage suggests a wiring problem, and you’re probably going to need that access to troubleshoot the problem.

Offhand, sounds like a problem in the pump/fuel sender harness. Considering the age of the car corroded connections are quite possible.

If the car were mine and I were planning on keeping it for a while, I would replace the pump assembly along with repairing any wire connector problems that may exist.

This is especially true if the pump has never been replaced and being pro-active on fuel pumps is one thing I’m kind of a stickler for because I don’t like being stuck in the middle of nowhere (has happened anyway) at an inopportune time.

The fuel pump and sender use a common ground back by the tank which would be my first suspect.

I think Pete will win this one.

Should everything external to the tank check out, be aware that there were some problems a while back with excess sulpher is gasoline causing electrical problems (see attached link).
I’m not suggesting you chase the suit, only be aware of the issue in moving forward with your diagnosis.

Thanks guys! This has really been helpful. I’m going to check the ground first, and then I’m going to get a voltmeter (it’s a good thing to have) to run some tests.

There is no such things as a problem that can’t be diagnosed.

Oh,oh! Wait a minute. OK has one, unless he fixed it, and I missed it. Sorry.

In response to Irlandes kidding shot to OK4450 I would say most good techs have run across at least one problem during their time working on things that leaves your head shaking.

In regards to this problem I too would vote for the grounding to the tank being the cause of the problem. Running a temporary ground jumper across the suspected bad connection area could prove that.

Unfortunately, irlandes is correct. My SAAB still sits and I’m just as clueless now as I was 6 months or a year ago. What a travesty. :frowning: