1987 GMC Suburban MAP Sensor shuts down engine

Tiny
CASPERSMITHS
  • MEMBER
  • 1987 GMC SUBURBAN
  • V8
  • 4WD
  • AUTOMATIC
  • 137,926 MILES
Initially the suburban would start missing and stalling and the problem got worse until it would not run.

I was investigating the problem and discovered that the engine idled good when the MAP sensor vacuum line input was not connected to the vaccum line from the backside of the carburator.

I have replaced the map sensor, the EGR valve and EBR solenoid and the problem remains.

what else would cause these symptoms?
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Monday, December 21st, 2009 AT 6:08 PM

40 Replies

Tiny
BMRFIXIT
  • EXPERT
Remove the TBI unit and check vacuum passages under its gasket as it may be clogged and giving the wrong vacuum reading causing your issues
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Monday, December 21st, 2009 AT 6:30 PM
Tiny
CASPERSMITHS
  • MEMBER
Do you mean the carburator?
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Monday, December 21st, 2009 AT 6:40 PM
Tiny
BMRFIXIT
  • EXPERT
Yes but we call it TBI
As it an injection system
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Monday, December 21st, 2009 AT 6:43 PM
Tiny
CASPERSMITHS
  • MEMBER
I will check and get back to you
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Monday, December 21st, 2009 AT 6:46 PM
Tiny
BMRFIXIT
  • EXPERT
Let me know
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Monday, December 21st, 2009 AT 6:50 PM
Tiny
CASPERSMITHS
  • MEMBER
I removed the TBI and made sure there were no blockages (there weren't) and put it back on and started the Burb with the MAP sensor vaccum plugged in.

The Suburban has 15 inches of vaccum at the manifold and the engine idles rough.

The engine lopes (surges and falls off) while idling and then dies within a minute.

I can keep the engine running at 2000 RPM with the MAP sensor plugged in.

Could this be a vaccum leak problem?
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Tuesday, December 22nd, 2009 AT 2:02 PM
Tiny
BMRFIXIT
  • EXPERT
Check timing
loosen distributor and turn a bit
connect all that was disconnected
clean throttle body and idle air control valve
disconnect battery wait and then reconnect
start engine and let it idle
drive it around if possible
if none help
check for codes if any
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Tuesday, December 22nd, 2009 AT 6:22 PM
Tiny
CASPERSMITHS
  • MEMBER
I have done these things already. What would make the suburban run poorly with the MAP sensor connected?
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Thursday, December 24th, 2009 AT 1:27 PM
Tiny
BMRFIXIT
  • EXPERT
Are you getting any codes?
Wrong vacuum
bad sensor
timing
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Monday, December 28th, 2009 AT 6:23 AM
Tiny
CASPERSMITHS
  • MEMBER
I have checked the emissions control sensors and they all check out.

The timing was just set.

What do you mean by wrong vaccum and how can that be fixed?
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Monday, December 28th, 2009 AT 12:43 PM
Tiny
BMRFIXIT
  • EXPERT


http://www.2carpros.com/forum/automotive_pictures/99387_Graphic1_628.jpg



This is how the MAP sensor work and when the ECM depend on it
Manifold Absolute Pressure (MAP) Sensor
The MAP sensor is a variable resistor which measures the changes in the intake manifold pressure which result from engine load and speed changes.
The pressure measured by the MAP sensor is the difference between barometric pressure (atmospheric air) and manifold pressure (vacuum).
A closed throttle condition (engine coast down) would produce a low MAP reading while a wide open throttle condition (engine acceleration) would produce a high MAP reading.
The high value is produced because the pressure inside the intake manifold (vacuum) is the same as the pressure outside the manifold (atmospheric air).
The ECM supplies a 5-volt reference signal to the MAP sensor.
As MAP changes, the electrical resistance of the sensor also changes.
By monitoring sensor output voltage, the ECM is informed of intake manifold pressure.
A higher pressure (high voltage) requires more fuel, while a lower pressure (low voltage) requires less fuel.

SO ,When the engine is running above 400 RPM, the ECM operates in the open loop mode.
In open loop, the ECM calculates injector pulse width based upon coolant temperature and manifold absolute pressure.
The engine will remain in open loop operation until the oxygen sensor reaches operating temperature, the coolant temperature reaches a preset temperature, and a specific period of time has elapsed after the engine starts. When all these conditions are met, the ECM operates in the closed loop mode. In closed loop, the ECM controls injector pulse width based upon oxygen sensor signals to maintain the air/fuel mixture ratio
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Monday, December 28th, 2009 AT 9:34 PM
Tiny
CASPERSMITHS
  • MEMBER
I just measured 17 inches of vacuum from a port off of the intake manifold.

When I try to drive the Suburban it won't accelerate but idles great
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Monday, December 28th, 2009 AT 10:29 PM
Tiny
CASPERSMITHS
  • MEMBER
Thanks for the info. So the MAP Sensor only controls the engine performance in open loop mode which occurs before the Oxygen sensor heats up to 600 degrees (as per my Suburban Haynes manual) and then the Oxygen sensor controls the performance.

Something is giving the MAP sensor false readings that will not allow the engine to accelerate under a load (it will accelerate in park though).

With 17 inches of vaccum I am not getting bad vaccum readings.

This week I warmed up the engine real good and the performance was terrible. I will replace the oxygen sensor and see what happens since its a $20 part.
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Tuesday, December 29th, 2009 AT 11:08 AM
Tiny
MERLIN2021
  • EXPERT
With onlt 17hg of vacuum check for an exhuast restriction, keep the vacuum gauge on the manifold, raise the idle to 2000 RPM, watch gauge, if it starts to fall, the exhaust is restricted. Or it could have weak valve springs, compression check the motor.
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Sunday, January 3rd, 2010 AT 12:13 PM
Tiny
CASPERSMITHS
  • MEMBER
Yes the vaccum fell dramatically. The heads are brand new so you can rule out springs and the compression is 130 to 150

The exhaust system is old and it has dual cats which I am going to remove since Wyoming doesn't have emissions laws to conform to.I'll keep you posted
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Thursday, January 7th, 2010 AT 5:32 PM
Tiny
BMRFIXIT
  • EXPERT
Just a note : Catalytic converters is a federal law and not just a state
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Thursday, January 7th, 2010 AT 6:30 PM
Tiny
MERLIN2021
  • EXPERT
Yes. Keep us posted I would like to know the outcome, clogged cats will give your symtoms, even a crashed pipe can. BMRFIXIT is right about Fed emmissions, Cats are required by Fed law. Do what you want, but if the state has an inspection sticker program, Fed law supercedes all!
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Friday, January 8th, 2010 AT 2:43 PM
Tiny
CASPERSMITHS
  • MEMBER
I checked and Wyoming doesn't have a program for enforcing CAT compliance yet.

The CATs were badly plugged and removed and the engine can breathe now.

When I connect the vaccum gauge to the intake manifold the vaccum is at 16 hg and when I increase the RPMs to 2000 the vaccum no longer drops it stays at 16 hg.

I timed the engine at 0 degrees and it will start and idle rough so I increased the timing to 8 degrees advance and it idles and starts easily. The engine will accelerate to 2000 max under a load.

The engine acts like it wants to accelerate but won't.

It reminds me of a engine timing that won't advance.

What do you recommend? Would a hidden vaccum leak not allow the engine to advance and accelerate?
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Tuesday, January 12th, 2010 AT 12:22 PM
Tiny
MERLIN2021
  • EXPERT
If the cats were badly clogged, take mufflers off and check pipes, these chunks that break off cats can lodge anywhere in the exhaust, if you can remove as much of the system as possible and test drive it. You will find more, especially in muffler.
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Tuesday, January 12th, 2010 AT 1:55 PM
Tiny
CASPERSMITHS
  • MEMBER
I removed the entire exhaust system and did find some peices of of CAT in it and then ran it but there was very little difference in performance.

What to check next?
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Wednesday, January 13th, 2010 AT 5:43 PM

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