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92 740T Stall Report and Notes 700 1992

posted by duce1 on Fri Mar 30 11:21 CST 2012
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last visit: Mon Apr 2 17:45 CST 2012

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Hey folks!
My 92 740 turbo wagon with 254k miles is having stalling problems. I've been trying to convey information cleanly and accurately with other projects I'm involved in and I will attempt to do the same here.
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Symptom Overview:
-While fully warm and at driving speed a "bogging" fuel or spark type stall occurs. The stall feels progressive as though I go from 100% power to 50% then quickly look for a spot to die with remaining 20% before its game over. It will try to start again and it briefly fires but its acting fuel starved (could be wrong assumption though)
-If its cool I can restart and go a ways further but that's becoming less reliable quickly. Yesterday I got about 1/2 mile from my house and died. Its been cooling there overnight because frankly we both needed it.
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Initial Diagnostics and Relevant Issues:
- I had it towed to my friends shop after the first failure last week. I didn't know it was an intermittent issue and I was dead on the freeway. First actual tow truck in all my years of driving (tow ropes are in a different class IMHO).
- - Their diagnosis?: out of gas - this is true AND possibly false. (i.e. a
red herring)
- - - I might have been siphoned because I always zero out my odometer when I
fill up. My fuel gauge has been unreliable lately, still showing empty
when I just filled up. Odometer reads 80mi on new tank (approx 250mi
average) but they said they added 3 gals and it ran fine. Weird...
- - - - I told them I wasn't convinced (it just didn't feel right). They kept
it for code checks and a day of parts runs around town. Not a single
problem occurred and the tech even complimented me on the running order.
------------------------
After a few more fun and exciting stalls, I pulled it into my garage for my little regular maintenance routine.
- Clean mass air sensor, flame trap, idle air control valve, throttle body,
etc (typical old engine oil blow off and grime)
- Check and clean plugs, grounds, sensor connections, etc (electrical
system simple stuff)
- Check hoses for leaks, clean nipples at intake manifold, turbo lines cleaned,
etc (air flow and pressure stuff)

It started fine, idled with very minor fluctuation and overall ran like a champ. On the road test it died 1/2 mile away, and there it sits right now.
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Possible Diagnosis:
Fuel Delivery?: - Unlikely IMO - In the last 15k miles I've replaced in tank
and rail pumps along with filter and fuel pressure regulator. No
fuel leaks detected anywhere.
Ignition?: - Seems like a more reasonable guess. Its acting like "hot start"
but its a quickly degrading version. Replaced cap/rotor,
alternator, plugs, battery, coil, fuel pump relay, in the last 10k
miles. Unlikely culprits IMO.
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Goals Today:
Get the bugger home. Try for stall in garage and test fuel pressure, spark at plugs, test coil resistance.
---------------
Summary:
This is a great car when it runs and I can fix almost anything on it here in my garage/work shop. If I had more money I would gladly pay somebody else to fix or throw parts at it, but that's not really an option. I am a single Dad and busting my ass to make ends meet, so this car has to last a few more miles and I believe it can. I'd also like the dammed thing out of my shop because I have work to do. Thankfully more experienced people feel the way I do about these old bricks and I hope that someday I can return the favor.
Thanks for reading, the floor is now open...
Bruce

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92 740T Stall Report and Notes 700 1992

posted by Will740turbo on Sat Mar 31 00:09 CST 2012
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last visit: Wed Oct 22 16:09 CST 2014

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I think it might be the Crank Position Sensor. Have you checked yours? Black wire connector running from the firewall near the back of the cam to the bell-housing. Often the external insulation on these fails, but stays attached. That, along with the individual wires' insulation make for intermittent running, losses of power, and funny idles.
Also, next time it dies by the road, you might do a spark/fuel pulse check. For spark, a timing light would be awesome, but you could also just put a metal screwdriver in one of the plug wires so it shorts to the block and you can see while someone cranks the engine over. For fuel, a cheap "noid" light can flash to tell you if the injectors are pulsing. Either of these will give you more information, and there may be a place to attach a starter button (Make sure it's not in gear!) under the hood.

Good Luck!

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92 740T Stall Report and Notes 700 1992

posted by duce1 on Sat Mar 31 13:33 CST 2012
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last visit: Mon Apr 2 17:45 CST 2012

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Thanks Will,

IPD* said the same thing about the crank sensor just the other day. There will be some investigation there for sure.

I have a timing light on loan and admittedly Ive never used one before. My neighbor was discussing the same procedure you brought up and he is going to do a run down with me this weekend.

Hearing the same ideas from more than one source is quite reassuring actually. Im on the learning curve here and keeping my ears open.

Bruce (#2)

* http://www.ipdusa.com/

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92 740T Stall Report and Notes 700 1992

posted by someone claiming to be fixit2002 on Fri Mar 30 13:39 CST 2012 [RELATED]

The relay is a good bet although it usually doesn't cause a progressive type problem like you described. It's usually there or not. You can check it by disconnecting all the injector plugs and turning on the ignition. One connector on each plug should have 12 volts to ground and the other has zero. If you do get 12 volts try taping on the relay to see if it fluctuates. If you're handy with a soldering iron you can try touching up the joints in the relay. Another thing that I've seen mentioned hear that srot of mimics your problem is reports of a crack in the gas tank pickup tube that only causes a problem when the fuel level drops down below the level of the crack. Another thing to concider.

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92 740T Stall Report and Notes 700 1992

posted by duce1 on Fri Mar 30 14:18 CST 2012
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last visit: Mon Apr 2 17:45 CST 2012

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Thanks fixit2002,

"I've seen mentioned hear that srot of mimics your problem is reports of a crack in the gas tank pickup tube that only causes a problem when the fuel level drops down below the level of the crack. Another thing to concider. "

Thats a very good point actually. I try to keep about half or better in the tank. Its also better for the in-tank pump I have heard. The problem at this point doesnt completely indicate that on the facts but its a red flag to keep in mind.

It would be wonderful if it was something so simple as has been mentioned here by you and Bruce.

Shooting high and keeping my expectations low.

Bruce (#2)

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92 740T Stall Report and Notes 700 1992

posted by lucid on Fri Mar 30 13:10 CST 2012
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last visit: Fri Oct 24 12:31 CST 2014

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I had a very similar fault with my '93 940 Regina. See symptoms and cure here.

Turned out to be an intermittent failure of the RSRelay coil ground wire.

The RSR supplies voltage to the injectors on my Regina FI. Same for your turbo (+12 to injector resistor pack) -- plus power to Coil 15 on your turbo.

The RSR ground on turbo is left side inner fender, close to headlight.

As a quick test you can put a jumper wire in the RSR harness plug, bridging the 2 fat wire plug sockets. There's battery +12 on the Red wire, but the Green one is open until the ECU grounds the injectors

--
Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

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92 740T Stall Report and Notes 700 1992

posted by duce1 on Fri Mar 30 14:11 CST 2012
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last visit: Mon Apr 2 17:45 CST 2012

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Thanks Bruce,

I read that entire thread and it sounds about right.

My neighbor and I got it home. I'm ashamed to say the last stall is definitely my fault. In my haste I didn't tighten the ring clamp at the manifold intake tube near the battery side and it popped off. The sensors couldn't make heads or tails and so it would just poop out after 5 seconds. It was stupid on my end.
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Ive learned honesty is a must when solving problems and pride can easily be a liability just like ignorance. Owning my F' up there.
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Current state:
I just ran it for about 30 mins in low gear through my hilly neighborhood to get full running temp and rpm's similar to actual driving speed. No problems so far.
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Next:
Follow RSR jumper test and go over that area again.
----
Thanks for the tip Bruce. Ill report back with my findings hopefully tonight. Parenthood demands my attention right now...its a war of attrition folks.

Bruce (#2)

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