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Old 02-01-2012, 03:52 PM   #1
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Default 1994 olds 98 Diagnostic Dilemma

Inherited this car when my father-in-law passed away.. and it'* run great.

About 2 months ago, the ck engine light started coming on randomly, but there were no other symptoms.. a few weeks later, it started missing at times, then progressed to stalling. I changed the plugs and wires, put a new fuel filter on it, cked for spark, cked for power to the injectors, cleaned the MAF sensor, cleaned the throttle body, cleaned the battery terminals, ran some heet through it, tried a higher octane fuel... only to have it happen again.
It was all random. It might drive fine for 10 miles.. then ck engine light would come on and it might do nothing more than miss a few times, or it might really buck and then stall.. always would restart, and then it might be fine again for quite a while. After a while I figured out that it was always worst after the tank has just been filled. In fact, the symptoms are often completely gone once you've got about 75 miles on the tank.
I took it in for diagnostics and trouble codes indicated MAF sensor and crankshaft position sensor. I had already cleaned the MAF twice. They were sure it was the CPS, so I had it replaced.. no difference.
The repair shop spent a full day driving it and running diagnostics.. but couldn't get it to act up only rarely and finally gave up on it. At least they didn't charge me for replacing the CPS, because it didn't solve the problem.
I drove it home without incident (shop was 40 miles away), and it ran fine for two days.. no problems. Then filled it up one night and the problems started again the next morning.
I was thinking fuel pump, but wouldn't that be more likely to act up when the tank was low rather than full? EGR? It never threw any codes for fuel or exhaust system when they were trying to diagnose it. Why would it have indicated MAF and CPS when those were obviously not the problem?
My kids normally drive this car to school.. and we really need it back. Any help would be greatly appreciated.
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Old 02-08-2012, 10:10 AM   #2
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Thanks for all the help, guys.
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Old 02-08-2012, 10:29 AM   #3
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What codes was/is it showing?

What is the fuel pressure?

Other than the miss, when it is running is there any indications of a lack of power?

My guess is the fuel pump but we need more information to be more conclusive.
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Old 02-08-2012, 10:30 AM   #4
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And by the way, a simple "bump for response" is just as effective and is a more polite way to get some attention.
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Old 02-08-2012, 12:36 PM   #5
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My apologies. I'm not familiar with forum etiquette.. and just a little frustrated.

I don't know the actual codes, but they indicated MAF and crankshaft position sensor. I asked about fuel pressure and the tech said they had cked fuel pressure and it was wnl. I asked if they had cked the evap system.. and he gave me a blanket answer that "we didn't pull any codes on the fuel side." I take that to mean they didn't physically check it.

I did find a reference to a tsb (01533) issued by gm about starting and stalling after fuel fill up, without any dtcs. I couldn't find any more than that on it than that.

I took the car in to another shop this am for more diagnostics. It'* about 15 degrees out. The car stalled 3 times on the way to the shop and bucked quite a bit. The ck engine light was on the whole time. It'* still reading above full on the gas gauge. They should be able to pull codes on it at least. I kind of wish I had changed the charcoal cannister filter in the evap system before taking it in.

Thanks for your response. I'll post whatever I find out from this shop.
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Old 02-08-2012, 09:52 PM   #6
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I talked to the tech at the shop.. it'* still setting codes for the crankshaft position sensor.. even though I just had that replaced. He thinks it'* what he called the "tone ring" that the sensor reads, and said it was off position by 40 degrees.

But when I asked him why it only happened when the tank was full.. he agreed that it didn't make much sense. And I know the tech that replaced the cps has 20 years in and I can't imagine him not cking the parts related to the cps when they continued to get that code a time or two after replacing it.

Here'* how random this thing is.. when I took it to the shop this morning it stalled 3 times and missed a bunch. The tech said it died on him 3 times getting it into the shop from the parking lot.. but then when he brought it out for me to pick up.. it idled fine while we talked for a while, then drove fine all the way home.. no problems.. I don't know if it'* because the gas is down a bit further now or if it could be something about having it hooked up to the diagnostic tools that changes the problem. At the last shop, the more they worked to diagnose it, the better it got.. then ran great all the way home (40 miles or so), but got real bad after the next fill-up.

Doesn't make sense. Maybe there are two problems.. and one still does involve the cps.
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Old 02-09-2012, 08:16 AM   #7
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i wonder if you got some trash on your fp pickup screen and when you fill it up it stirs up the trash and gets caught in the screen, then sometimes it would settle. or some of the rubber tubes they put in the tank are getting soft from the bs gas we have now, maybe its randomly pinching off. do you have a pressure gauge you can stick out the back of the hood so you can watch while driving?
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Old 02-09-2012, 08:58 AM   #8
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Dificult to come up with ideas, particularly when mechanics who actually can play with the car can't figure it out. Thinking about why this could happen when the tank is full, I am wondering if the tank is not getting pressure relief. Just a randon thought, but try running with the gas cap off next time it is stalling.

As far as crap getting stirred up in the tank, I would think it more likely to get stirred up when the tank was low than when high.

I know you said the mechanic said fuel pressure was okay, but do you know what the pressure reading was?
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Old 02-09-2012, 10:08 AM   #9
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The tank now has about 48 miles on it. It'* fairly cold again this morning (20) and I drove it to work. No ck engine light.. not one miss.

I have cked the gas cap to see if there was any vacuum on removal.. but haven't tried with it loose or off. I can do that.

I've seen some forum posts online that talk about the evaporative system, the charcoal cannister, and an evaporative solenoid. I located the cannister. I'll ck that and track the lines to the engine and solenoid and see what I can find. That post said that if the solenoid was stuck open, it would be a problem with a full tank.. a solenoid would normally be open at rest, right? So if I can disconnect the wiring to the solenoid.. would that allow me to test it while leaving it closed?

Thanks.
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Old 02-09-2012, 10:12 AM   #10
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Oh.. and with respect to the pressure gauge while driving.. this last mechanic had suggested that too. I have a gauge.. not sure if it has a connection for the schrader valve that'* supposed to be on the fuel system, but I can probably rig one up. I was wondering though if the problem, even if it wasn't fuel pressure directly, wouldn't show a "bump" w/ missing.. due to back pressure.

I didn't ask what the fuel pressure actually was. I may get to that myself later today if I can get my gauge hooked up.
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