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1991 Cutlass Supreme. 3.1 Engine Code 34 confusing me and my…

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1991 Cutlass Supreme. 3.1 EngineCode...
1991 Cutlass Supreme. 3.1 EngineCode 34 confusing me and my local mechanic.Had a problem with a fuse constantly blowing and when I seemed to have fixed that, IMMEDIATELY I began to have a problem with my car stalling out with a code 34. IF I would unplug the MAP sensor, I can manage to get where I am going and at least not be stranded, but that is no way to run the car. Idle Air valve had a bad O-ring, replaced that, still erratic behaviorMechanic and I are both befuddled.
Submitted: 11 months ago.Category: Chevy
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Customer reply replied 11 months ago
Last night,I drove it to my mechanic and could not get out of the driveway before it stalled.... I went to unplug the MAP sensor, and it was dark and I accidentally unplugged the solenoid. Drove all the way with no CEL. If I unplug something, it will run, otherwise, it will stall out.I kept blowing the 15 AMP IGN fuse within three miles or less of replacing it. I unplugged EVERYTHING and would still blow the fuse. Car would run, with no problem but there would be no AC and no electrial connection to the EGR, solenoid or voltage regulator.
Finally I noticed a couple of pine needles in between the intake manifold and the power steering pump Brushed those out of the wiring harness, and have no problem with that fuse.. But the very next day is when this stalling out started.
Providing this information for full disclosure to see if this could give a clue as to what is the problem.
Answered in 1 hour by:
9/22/2017
Chevy Mechanic: Jerry Newton, Chevy Technician replied 11 months ago
Jerry Newton
Jerry Newton, Chevy Technician
Category: Chevy
Satisfied Customers: 5,694
Experience: ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
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It seems likely that, since you moved that wiring and you're now getting a code 34, there's something wrong with the MAP sensor signal, likely having to do with a chafed wire in that harness. If you had a chafed wire that was making the fuse blow, there is probably another chafed wire adjacent to that.

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Customer reply replied 11 months ago
We haven't access to an OBD1 scanner, but the mechanic unplugged the MAP sensor as the car was running and noticed the change so he assumes that the MAP sensor is running properly, so the assumption is that the wiring is correct. Unless the pine needle cut most of the way through that particular wire.
Chevy Mechanic: Jerry Newton, Chevy Technician replied 11 months ago

If you disconnect the MAP sensor, the computer recognizes that it's disconnected, and runs the engine using backup tables. However, if the MAP sensor is connected and a short or open is affecting the signal, the computer can't decipher that mixed up signal from a real signal, so I don't agree that just because you unplug the MAP and the problem goes away means the MAP is good. The MAP sensor itself probably IS good, but I suspect a wiring issue. The wiring on this engine runs under the upper intake, and it's very prone to damage because of the way it's made. I would definitely be looking at the MAP circuits visually, even removing the upper intake if necessary to get a look.

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Customer reply replied 11 months ago
I am confused how the car would know that the MAP sensor is disconnected. One wire is just a ground, another wire gives a 5 Volt power and the other is the actual signal. If the signal wire is an intermittent open, wouldn't it just use the backup tables like you said, and not stall out?There is a lot of sludge, but if I am not mistaken, these three wires, closest to the fuel rail, are the three wires going to the MAP sensor. They look like the correct colors. Granted, they are not the only wires going under the plenum. The EGR and the Solenoid both run the same route and are both on the same circuit with the fuse that blew.
Customer reply replied 11 months ago
A clerk at Autozone had told me that someone had a similar problem but it wasn't the MAP sensor but he couldn't remember what it was but when they fixed that the MAP sensor worked correctly. The only thing that I haven't changed yet is the TPS.
Customer reply replied 11 months ago
I didn't have an old MAP sensor to test with, so I put a new MAP sensor on the car - wish I hadn't as those are expensive - so there is no problem with the MAP sensor itself.Earlier this week to test it, I started the car in the first time in a week and drove it around a twelve mile loop. No problem, no engine lights, nothing. Checked the codes, and no engine codes stored. Waited ten minutes, started the car again, engine light comes on immediately. So the problem really occurs when the car gets to operating temperature, and then I make a stop somewhere and then start it back up again a few minutes later when the engine still warm.
Customer reply replied 11 months ago
The schematics show that the MAP sensor wires goes to a C102 connector. My manual only shows the location of a C102 for a VIN D and I have a VIN T.If that circuit is indeed the problem, it would seem that the easiest way to fix it would be to just get some 20 AWG wire and thread it through the plenum and just run it directly to the C102 connector. In fact, that may be the easiest way to trouble shoot if a mechanic has the right tools to re-set the electrical connections.
Customer reply replied 11 months ago
Sorry, something that I wanted to ask from the start. What telltales are on this car?? In other words, when does the CEL come on when there is something other than a code.I know that there is a telltale when the engine is critically overheating, but what other telltales exist? Sometimes I will drive and the CEL will come on, but when I check for codes, there are none there.
Chevy Mechanic: Jerry Newton, Chevy Technician replied 11 months ago

Sorry for the long delay, I got called away from my desk for a bit.

The ECM puts 5v and a ground to the MAP sensor, and then watches the third wire for a signal. If the signal wire goes open, or if it shorts, the computer recognizes it and sets the code. 34 is a generic code for the MAP sensor circuit, having a scan tool would go a long way toward figuring out what's wrong because you could see the actual signal.

Yes, if you suspect that the wires ARE damaged, you can just run new wires. It's not that much work to remove the plenum, but if you want to avoid it, you surely can do so.

Aside from your typical charge, oil pressure, and coolant temp lights, the CEL is the only telltale. And it's not like an OBD2 car, the light resets after each key cycle, so if you have a failure, it has to fail in real time to make the light come on, and more often than not, the light will go out if the failure condition goes away.

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Customer reply replied 11 months ago
The error codes are stored in the ECM unless the 20 AMP fuse is removed or the battery is disconnected. Otherwise I wouldn't be able to check them with a bare wire in the diagnostic port, because that won't work if the engine is running. Or at least, that is how it is supposed to work.I know that the Check Engine Light will come on if the coolant temperature goes off the scale. Personal experience but there is no diagnostic code for that. I wanted to know what else would cause the Check Engine light to illuminate without a corresponding code as that might give another clue as to what is happening.My mechanic will not be available until Monday to discuss running a new wire or whatever. i may, over the weekend, try wiggling those wires while it is running to see if that makes any changes. It could be that when I unplugged the solenoid, that I pulled on the other wires and it was positioned to a place where it had a good connection. I don't know at this stage.
Chevy Mechanic: Jerry Newton, Chevy Technician replied 11 months ago

If the check engine light comes on, there's a trouble code stored. It's true, you cannot check codes with engine running using the AB jumper technique. An old scan tool would do it, and I'd probably try to source one. I have a generic OBD2 scan tool that will read OBD1 GM, Ford, and Chrysler, and I think I spent all of 30 bucks on it at a parts store some time back.

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Customer reply replied 10 months ago
Sorry for the delay.We are in the middle of nowhere, so I did not have a ride back until late yesterday. I drove it to the mechanic with no incident, but when he tried to move it, it would stall out.That has been the issue. It would run fine, but if I shut off the motor and wait ten minutes and restart it at operating temperature, the CEL will come on and it will stall out when putting it in gear or pressing the accelerator.I cannot figure out how that would make such a difference.
Chevy Mechanic: Jerry Newton, Chevy Technician replied 10 months ago

If it's truly a no codes situation, and it runs/stalls/barely runs with a CEL for no apparent reason, I would have to say that I'd put a computer in it.

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Customer reply replied 10 months ago
The mechanic glued the three-way coupler onto the interface at the throttle body and all the Check Engine Lights and the stalling quit. I am not sure how a leak at the throttle body would cause a Code 34. But those all stopped.The issue now is that that 15 AMP IGN fuse has blown, which I thought I had fixed just before this Code 34 started. Not sure if these are related issues or not.
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