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Discussion Starter #1
:banghead:

I have been having a reoccurring problem with my ride. I have a 1985 Deville, in damned near mint condition. First car with the on-board computer I'm told. I got it from a deceased relative who had used it as his Sunday car since 1993. It means a lot to me.

I am getting the amber Service Now light along with a E51 Code. PROM error. I replaced the PROM, then the ECM. The light stayed on for a week, but the cars performance improved dramatically.

The problem started three months ago. I was driving down the freeway at a constant 60. The car gave a little shutter and the Service Now light came on. I took it into a reputable shop. I asked for a diagnosis only. The GM says he can't figure out the problem.

Symptoms:

- High Idle
- Rough Shifting form P to R to D
- Stutter or Stall When Cold

The light would go on and off intermittently in the beginning. It stayed on for a week, then clicked off. I could feel the difference immediately. She began behavin again. Then a few weeks later, on again. After a while it stayed on.

I am not much of a car person. I replaced the ECM after a lot of learning. The car started to shift like butter. I waited the 50 turns of the key and the light went out. Honky dory. This was a few days ago.

This morning I am headed to a meeting that is a good distance away. Once again, I am pegged at 55 , I feel the shutter and the light pops on. I leave the meeting four hours later and the light is off. Halfway home, about the same distance as this morning, Boom, On Again!

I need Qualified, Experienced Advise.
 

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Are there any current codes? I'd be suspect of several things:

1. Ignition system. Intermittent issues can sometimes tie in with climactic changes, especially precipitation.
2. MAP sensor. An intermittent MAP issue can cause the very issues you describe. You might want to try to drain the MAP line of condensed fuel.
3. Simple mechanical issues like vacuum leaks, bad electrical connections etc.
 

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'80 Fleetwood Coupe, 1994 and 1995 Mercedes 140 Coupe
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I'd give the idle speed control motor and switch a good look. They don't always set a code when they flip out.
The service manual trouble trees are a real help.
 

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2003 STS w/BoseNav, 1993 Deville, 1985 Deville
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Hmmm...I wonder if the connectors at the PCM and BCM are loose or somehow got dirty?
 

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88 Sedan Deville, 89 Eldorado, 11 SRX, 18 CT6
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130 Posts
straight up highway speeds eh?
well... on my 88, at 55-68 the EGR valve was "sticking" and caused very similar conditions. replaced that and it all went away.

on the 1990 fleetwood, at 45-70 it runs awesome, and then stutters, then throws an EGR error and AIR error.

my two cents :)
 

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Discussion Starter #6
Thank ya'll. I will look into all of these things. There is one other syptom that I failed to mention.

The digital gas gauge is on the fritz. The miles til empty meter is continuely in flux, exccept when the big L pops up. The L will pop up and stay for a while. Then it will start at 40 miles til empty and gradually work it's way up to 999 then L agian. Sometimes it will reflect how many gallons I have. If I have 7 gallons, the meter will read 764 or something. Then when it gets down to 6 gallons, the meter reads something like 658.

When the Service now light goes off the gauge works fine.
 
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