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MAP sensor, runs better unplugged

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Old 10-23-2007, 08:33 AM   #1
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MAP sensor, runs better unplugged

(Different problem then FPR, so I thought I’d start a new thread.)

88 S-10 Pickup, 2.5L, 5-Speed, 130k miles.

Truck will only start if I put fuel down the carb. Then, it sputters, spits and coughs, and will only stay running if I pump the gas pedal. No codes on the SES light. When I unplug the MAP sensor, it smooths right out, and will even idle on its own for several minutes until it finally dies. With the MAP sensor unplugged, I get SES code 34 (bad MAP sensor of course).

So I put on a brand new MAP sensor. Same problem, sputters, spits, coughs. I then unplug the MAP and it runs okay again (until finally it dies after a few minutes)

Here’s a list of what I’ve checked and replaced:

Checked for vacuum leaks-none found
Removed Cat. Converter (in case it was clogged)
Checked for alternator loose connection (in case FP wasn’t getting enough power)
Fuel Filter-replaced
Spark plugs-replaced
PCV valve-replaced
Fuel pump relay-replaced
Dist. Cap and rotor-replaced
MAP sensor-replaced
Air box temp sensor-replaced
Air box manifold breather-replaced

What I have NOT replaced:

ECM
EGR Valve
IAC (Idle Air Control) Valve
O2 sensor
Fuel Pump
Wires

Also sprayed in some carborator cleaner, and put in some fuel stabilizer cause I thought the gas might be old. Also ran it down near empty then put it new gas.

I’m slowing running out of ideas and things to replace. This is driving me crazy! Anyone have any idea what could possibly be wrong?

Thanks in advance.
John

Last edited by JohnN; 10-23-2007 at 08:45 AM. Reason: corrected info
Old 10-23-2007, 09:50 AM   #2
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Re: MAP sensor, runs better unplugged

Everything is pointing to a bad fuel pump I think. Classic symptoms.
Old 10-23-2007, 07:17 PM   #3
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Re: MAP sensor, runs better unplugged

check the grounds for the ECM,they attach to the intake manifold. they'll cause your injector to not spray or spray intermittantly. also check the fuel pressure,should be around 15 psi
Old 10-24-2007, 07:51 PM   #4
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Re: MAP sensor, runs better unplugged

I agree. sounds like fuel pump.
Old 10-24-2007, 08:47 PM   #5
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Re: MAP sensor, runs better unplugged

Quote: Originally Posted by kmhowell01
I agree. sounds like fuel pump.
Okay, thanks for that input. I'll check those ground connections on the ECM. I can see most people say it's the fuel pump. I'll replace that too, but I wonder why it seems to run good when I unplug to MAP sensor. It seems to be getting plenty of gas....

I dunno ???

...

Last edited by JohnN; 10-24-2007 at 08:48 PM.
Old 10-25-2007, 02:15 AM   #6
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Re: MAP sensor, runs better unplugged

when you unplug it its going to revert to stored info, so it runs the engine rich but with low fuel pressure it can only spray that much fuel for a short time till it runs outta pressure
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