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boltz

'95 850 NA hot stalling problem

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I've been working on this for a couple of months. It only happens when the engine is thoroughly hot. The car starts instantly and runs perfectly for the first 20 minutes, with full power acceleration (well, full power for an NA...). Typically it shows up when I've been driving at highway speed and hit heavy traffic. After about 5 minutes of crawling in traffic it starts having trouble finding idle, dropping down in the 300 range and wandering up and down. Sometimes it dies, but always starts right back up. If I use the gas pedal I can rev it up to about 1500 and keep it going. Most of the time once I get out of the stop and go it gets back to normal, but lately it's started cutting out at highway speeds.

I've thrown a couple parts at it so far. I've changed out the fuel pump relay for a brand new Volvo brand one with no effect. Also swapped out the IAC with a used one. About 5 years ago the OE fuel pump died and I put in a Walbro. I can't find my right angle schrader adapter so I haven't checked fuel pressure yet. I will, but the pump is quiet and there's been no change in WOT performance. Not getting a CEL, but probably still need to check for codes, trying to figure out the best reader option. It doesn't have the built in reader.

Not really sure where to go with this, any ideas would be greatly appreciated.

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While I wait for my scanner to arrive, I decided to study up on OBD2 and especially mode $06, which can help diagnose misfires which don't set a code. I stumbled upon this Delphi video with some details on troubleshooting pre canbus Fords using mode $06:

 

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Heat soak causes the problem = electronic/electrical part, resistance gets high with increased temperature. I'd throw a TPS at it, too.

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I was reading live data from the MAF last night and the value spiked right before the idle became low and unsteady. I'm still trying to figure out if this is the cause or a symptom. There was no code set. Common sense would say if the RPM drops the MAF should be reading less flow, not more.

The cooling fan had cycled several times, so it was good and hot under the hood. I'm wondering if there might be an intermittent open circuit inside the internal MAF circuitry which opens up when the temp goes up. 

My recurring codes are:

EFI-433 rear knock sensor

EFI-444 accelerometer

EFI-445 air pump

Obviously the air pump can't cause a stalling problem, but I don't know about the other two, info on them is pretty thin. I'm going to look at ful trims next and see if anything stands out.

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