1987 GMC Suburban MAP Sensor shuts down engine

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

Do you mean the carburator?

yes but we call it TBI
As it an injection system

I will check and get back to you

Let me know

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?

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

I have done these things already....what would make the suburban run poorly with the MAP sensor connected?

Are you getting any codes ?
wrong vacuum
bad sensor
timing

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?

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

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

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.

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.

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

Just a note : Catalytic converters is a federal law and not just a state

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!

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?

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.

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?

Does it feel like lack of engine power, or lack of transmission shifting? High revs but does not go? Or not revving?

The engine is cutting out. When I push on the gas pedal the engine won't advance and sputters like its choking.

Is the check engine light on? I need a compression test done on this engine, remove all plugs and test this way, note results in each cylinder.
https://www.2carpros.com/dia/how_to_check_compression.htm
And a fuel pressure test wouldnt hurt either, but 16hg of vac is still too low, so either a valve spring, or severe vacuum leak exists...Still have that vacuum gauge?
https://www.2carpros.com/dia/test_engine_vacuum.htm
Then find vac leak like this. Get a can of spray Throttle body cleaner, start the car and spray around, pay attention to vacuum lines and manifold, if the idle changes, you found a leak. You want the engine running and spray on the outside but do direct the stream onto the hoses, if it is a vacuum leak, the engine idle will change speed, then you have detected a leak, repair the vacuum leak and see how it runs! If it has a vacuum power brake booster, disconnect and plug the hose and test vacuum with the gauge...Booster diaphram may have a leak too.
A good source on reading the gauge.
http://findarticles.com/p/articles/mi_qa3828/is_200108/ai_n8978956/

I am performing all that you need right now and will send the results when I am done

I was able to get the compression tests completed before the day turned cold.

Cylinder PSI
1 125
3 124
5 127
7 125
2 120
4 121
6 126
8 124

I will perform the vaccum leak test tommorrow as per your instructions

BTW....I own my vacuum gage so not to worry

Please re check engine timing, It sounds like the timing chain is off by a tooth or two, this would give all your symtoms. Place #1 piston at TDC, You can use a screwdreiver in the spark plug hole, and move engine by hand, this will giv you a visual on where TDC is, then check crank index mark, and distributor for correct positions.

The timing chain is a brand new double roller...it was replaced when the heads were replaced.

Although not impossible I was very careful to align the marks on the chain gears

TDC is where the piston reaches the top of his extension.....

Just a note when I start the engine it gives a loud hissing noise.....I have not checked for vaccum leak yet

This engine performed normally for about 6 months after the timing chain was replaced....It was last october of 2009 that the engine started having these symptoms

I think its a vaccum leak I just didn't have any way to find the leak until you explained your method to me

I just used the carb cleaner and sprayed all the vaccum lines and any location where a vacuum leak could occur and the engine did not show any changes.

I also plugged the power brake booster line and there was no change in vaccum.

I also moved the engine to zero degrees TDC and the piston is located as near to the cylinder head as it can go. The distributor is also aligning ok.

How much vacuum should this engine have?

I was going to take the suburban to a local mechanic when he informed that my operation of setting timing was incorrect.

How do you set timing?

He mentioned unhooking the relay with the brown wire and white stripe to set timing at 0 degrees.

I have never heard of this before

This is the proceedure for throttle body injection, you probably set it for a Carbureated model... Throttle Body Injection 1. Install timing light with an adapter between No. 1 spark plug and No. 1 spark plug wire or use an inductive pick-up. Do not puncture wire. Disconnect 1-wire (Tan with Black stripe) connector at distributor. 2. Check and adjust ignition timing with engine at normal operating temperature and transmission in "P" or "N" (auto. trans.) or Neutral (man. trans.) Reconnect 1-wire connector. Clear trouble code by removing ECM power fuse for 10 seconds.

I finally gave in and took the Suburban to the only local good mechanic I know of and will let you know what they find.

thanks for your help as we were able to get the Suburban to finally accelerate however it still cuts out on occasion.

The exhaust system being blocked and the bad oxygen sensor were the issues that I found with your help.

Update me when U can please!

The fuel pressure was at 6 psi and they found that the old fuel pump inside the tank was clogged and not working and was not removed before the prior owner installed the in-line fuel pump.

What a long strange trip it's been! Runnin OK now?

As soon as the work is complete we'll know if this is the extent of the problem

Replacing the fuel pump in the tank did the trick and the suburban is running good and all the symptoms are gone.

thanks for your help

Glad it all worked out, One question though, Wasnt this truck misfiring?

When I pushed lightly on the foot pedal the engine would run ok but once I pushed hard on the foot pedal the engine would hesitate for a 1/2 second and then die.