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Will not run with the MAP sensor vacuum hose connected.

Customer Question
Will not run with...

Will not run with the MAP sensor vacuum hose connected. Idles good when not connected but will not run when put in gear without working the gas peddle.

Mechanic's Assistant: How old is the transmission? Have you checked the brake switch?

Transmission came with the truck. Both front and rear seals leak and are to be replaced. The break switch seems to be okay.

Mechanic's Assistant: What is the model/year of your GM?

1994 Sierra 1500 pickup, 6 cyl

Mechanic's Assistant: Are you fixing your Sierra 1500 yourself? What have you tried so far?

I have replaced most of the sensors, the distributor, rotor, rotor cap, plugs and wires, fuel injectors.

Mechanic's Assistant: Anything else you want the mechanic to know before I connect you?

Also replaced was the throttle position sensor, ERG valve, fuel filter and TBI pressure regulator. With the vacuum hose off of the MAP sensor, I was able to set the timing TDC.

Submitted: 6 months ago.Category: GMC
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Answered in 1 hour by:
12/30/2017
GMC Mechanic: Pete, Mechanic replied 6 months ago
Pete
Pete, Mechanic
Category: GMC
Satisfied Customers: 139
Experience: Ase Master Tech
Verified

It sounds like the egr valve is stuck open. Is it a 4.3 engine? They had a habit of carbon pieces getting stuck in the egr valve. I see that you installed a new valve, but it can get clogged with carbon right after you start the engine. Take it out to see if a piece of carbon is holding the valve off of its seat. Tomco sell a special gasket with a screen on it. This stops the carbon from getting into the valve. Get back to me when you can.

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Customer reply replied 6 months ago
I removed the egr valve and it was seating properly. There were no carbon pieces in the valve. I did order the gasket with the screen and will install it when it comes in.
I still have the issue with the vacuum hose to the MAP sensor. Note that I did replace all of the vacuum hoses.
GMC Mechanic: Pete, Mechanic replied 6 months ago

Ok thats one thing eleminated. What sire is the engine? When you disconnect the map sensor do you leave the hose open or do you plug it? If you leave it unpluged and have a vacuum leak, I think the engine is running to rich. Do you have a scanner to check the engine sensors? Since it is a 94, iit is an obd1 system. The modern code readers will not work.

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Customer reply replied 6 months ago
It is a 4.3L engine. It will idle faster with the vacuum hose unplugged. I have it plugged and it idles good. I am able to rev up the engine while in park or neutral.
I checked the code by the check engine light. It gave the ECM code 12. (no distributor signal to Electronic Control Module; System PASS) Code 12 indicates the computers self-diagnostic system is functioning properly. No fault codes came up.
GMC Mechanic: Pete, Mechanic replied 6 months ago

Can you put a vacuum gage on the map sensor hose to get a vacuum reading?

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Customer reply replied 6 months ago
Vacuum on the map sensor hose is 20 in. HG vacuum.
GMC Mechanic: Pete, Mechanic replied 6 months ago

The vacuum looks good. When you plug it into the map sensor does it missfire or just run rough?

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