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1987 GMC Suburban MAP Sensor shuts down engine

Tiny
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Does it feel like lack of engine power, or lack of transmission shifting? High revs but does not go? Or not revving?
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Thursday, January 14th, 2010 AT 1:11 PM
Tiny
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The engine is cutting out. When I push on the gas pedal the engine won't advance and sputters like its choking.
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Thursday, January 14th, 2010 AT 1:23 PM
Tiny
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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/
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Thursday, January 14th, 2010 AT 1:33 PM
Tiny
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I am performing all that you need right now and will send the results when I am done
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Thursday, January 14th, 2010 AT 2:56 PM
Tiny
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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
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Thursday, January 14th, 2010 AT 6:52 PM
Tiny
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BTW.I own my vacuum gage so not to worry
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Thursday, January 14th, 2010 AT 6:54 PM
Tiny
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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.
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Friday, January 15th, 2010 AT 3:00 AM
Tiny
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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
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Friday, January 15th, 2010 AT 9:50 AM
Tiny
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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
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Friday, January 15th, 2010 AT 10:57 AM
Tiny
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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?
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Friday, January 15th, 2010 AT 2:48 PM
Tiny
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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
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Friday, January 15th, 2010 AT 3:01 PM
Tiny
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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.


https://www.2carpros.com/forum/automotive_pictures/62217_Specs_4.jpg

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Friday, January 15th, 2010 AT 4:37 PM
Tiny
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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.
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Monday, January 18th, 2010 AT 12:01 PM
Tiny
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Update me when U can please!
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Monday, January 18th, 2010 AT 2:47 PM
Tiny
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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.
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Monday, January 18th, 2010 AT 3:34 PM
Tiny
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What a long strange trip it's been! Runnin OK now?
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Monday, January 18th, 2010 AT 4:11 PM
Tiny
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As soon as the work is complete we'll know if this is the extent of the problem
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Monday, January 18th, 2010 AT 4:23 PM
Tiny
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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
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Tuesday, January 19th, 2010 AT 12:15 PM
Tiny
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Glad it all worked out, One question though, Wasnt this truck misfiring?
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Tuesday, January 19th, 2010 AT 5:06 PM
Tiny
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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.
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Thursday, January 21st, 2010 AT 11:38 PM

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