1991 GMC K2500 Engine problem

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Old 11-22-10, 09:15 PM
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1991 GMC K2500 Engine problem

Ok, here goes it will try to keep it to the point

1st attempt at emission tests failed.
So I changed the PCV valve, spark plugs, cap and rotor.
2 Spark plugs break of course, get them out. button things up.

Starts good, a bit rough but am sure the firing order was right.
Took a road test and was noticeably low on power, very slow to hit 60 MPH.
Come back and check firing order, again am sure its ok.

So... and this is where it starts to really go bad...
I decide to do a compression test, which actually showed 180 PSI on most of the cylinders pretty consistent. I was unable to pull #7 plug for fear of breaking, and I didn't want the grief of trying to extract a broken plug there. And #8 the compression gauge refused to thread in right, but it had lost of compression to blow out the gauge that wasn't screwed in well.
Thinking ahead, I pulled the connector off both injectors, and also pulled the high tension lead off the distributor.

Put it all back together and it would not run well at all, would not take any throttle or it would stall, any attempts to build RPM lead to backfires (and flames) from TBI, and a backfire thru the exhaust big enough to open the muffler up at the seem large enough to fit a soccer ball.
I suspect the CAT will be blown apart now ?

What could I have do to get the motor running this bad.
I thought it was the coil blown because it was arcing to ground during the test, replaced and no change.

Finally I removed the distributor advance lead under the fuse panel (used to set idle to TDC) and it ran enough to move it out of the garage.

One thing, it would run good for a second or two right after each time it was started.
A few hours later, I realized that when I was supposed to be using #8 wire I was actually using #7 and vide versa, but it ran that way prior to the compression test.

Have I smoked the hall sensor in the distributor, have i smoked the computer ?
It does not through any codes, other than MAP pressure exceeded (big backfires).

I don't know what to replace now, and this is my low truck and suspect I may just need it soon.

I can live with out valid plates, but not a low truck.

Any help would be awesome.

Thanks
 
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Old 11-22-10, 10:19 PM
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(Quote) 1st attempt at emission tests failed.

[COLOR="black"]Are you saying it failed the emmissions portion of the state [/COLOR]inspection? If yes post the print out or state the reasons for the failure.

Has the fuel filter been changed recently?

Post static, residual, and running fuel pressures?

Post the results of engine vacuum pressure test

Ignore the "MIL" Do a scan even if the "MIL" is off. Post any codes so we can see them.

Pull the plugs. In order. Post pictures of the electrode burns so we can see them

Rebuild the fuel pressure regulator if not recently done.
 
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Old 11-23-10, 05:25 AM
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Thanks for the response.
I do not have the results in front of me but it was high in HC at idle and 40 KPH (canada), the CO was high at idle and just squeaked by at 40 KPH.

The fuel filter has never been changed, I do not have the tools to get fuel pressures unfortunately.
I can get the engine vacuum next weekend (truck is at my property out of town).

Plugs were new, and when I pulled them for compression test they where sooted (black), to be expected with a messed up firing order it think.
I did a scan without he MIL off and thats how I found the MAP high.

I will reset the computer, fix the firing order, and plug in the timing bypass and run it.
I can then get new codes.
What is this about rebuilding the FPR ?

Thanks
 
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Old 11-23-10, 02:44 PM
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Post the actual “MAF” error code.

Clean the “MAF” with ONLY the product that appears in the link below. Be sure to follow the cleaning instructions printed on the can exactly.

CRC Industries Automotive Product Detail

Be prepared to possibly replace the “MAF”

Check the “MAF” as per the link below and post the results.

| Repair Guides | Fuel Injected Electronic Engine Controls | Mass Air Flow (maf) Sensor | AutoZone.com

(Quote) I do not have the tools to get fuel pressures unfortunately.

You must find a way to get them. Proper fuel pressure and volume is a fundamental basic that must be either ruled in or out

(Question) What is this about rebuilding the FPR ?

It's exactly that. The fuel pressure regulator is part of the "TBI" and needs to be rebuilt from time to time. If it hasn't been rebuilt recently, it should be rebuilt now. Go to a well stocked auto supply store and buy a rebuilding kit for the regulator. Follow the enclosed rebuilding instructions exactly.
 
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Old 11-23-10, 06:08 PM
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This motor does not have a Mass Air Flow sensor.
It uses a Manifold absolute Pressure sensor.

I Get the fuel volume and pressure, I understand fuel injection principals.

My real question is, what on earth could I have done by doing a compression test.
While the truck did not pull hard on the run before the compression test, it would not even idle after the compression test. I know the plug wires went back as they came off cause I marked them.
I did something, either screwed up the computer or the distributor or something to make for such a drastic change.
Would not grounding the high tension lead feed back and kill my distributer sensor. It could not have failed completely or it would not start unless there is a CPS (crank sensor) on this motor.
At this point I could care less about passing the emission test, I just want a functional truck back.
 
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Old 11-24-10, 09:28 AM
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(Quote) This motor does not have a Mass Air Flow sensor.
It uses a Manifold absolute Pressure sensor.


TOHeating,

Sorry about that
had a senior moment
was thinking MAP
but wrote and posted
MAF information


A compression test alone wouldn’t cause the high MAP error that is currently set. In the process of doing the compression test (you could have accidentally):

Damaged the MAP senor

Caused an electrical issue with the MAP (broken or disconnected wires)

Caused a vacuum issue with the MAP (broken or disconnected vacuum hose or connection.

Questions to answer to either rule in or out the above.

Did the MAP issue exist before the emissions inspection? (If yes, that’s reason for test failure)

Did the MAP issue occur after the emissions test, but before the compression test?

Click below to check the MAP

| Repair Guides | Fuel Injected Electronic Engine Controls | Manifold Absolute Pressure (map) Sensor | AutoZone.com
 
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Old 11-24-10, 03:41 PM
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I have no idea what the status of the MAP was before compression test, or even before the emissions test. There was no MIL lite, so I would not scan the computer.

I will test it Saturday when I can.
I still have a nagging suspicion that the ignition module in the distributer is screwed up, but I still get spark but they are not regular and stable.
 
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Old 11-25-10, 06:05 AM
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(Quote) I still have a nagging suspicion that the ignition module in the distributer is screwed up, but I still get spark but they are not regular and stable.

[COLOR="black"][I]Did you mess with the distributor, or try to turn it? Check the wires going to it. Make sure they are in good shape and none are frayed or exposed. Also check the wire connectors at the distributor are on tight and the pins inside the connectors are free of all corrosion. Corrosion causes oxidation which can impact spark delivery.

First chance you get post the ACTUAL high MAP error code.
 
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Old 11-28-10, 09:40 PM
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Ok, update.
Ignition module replaces, coil replace, cap, rotor wires replaced.
Spark plugs replaced.
Still F*** up.

Check MAP, voltage to it. in spec when engine off, drops when engine on.
Check TPS, voltage to it ok, resistance changes as the throttle is moved.

Check IAC does what it should.
NO CODES STORED.

Pulled computer, and find out its been chipped ADS Superchip 9113A i believe.

The really odd thing here is that when it is started with the timing bypass in normal mode it starts good and runs for a second, then you can hear the inlet to the TB change in sound. as if the IAC is opening to give it more air.

Manifold vacumn is low. Not able to get a good read on it but it's not stable and low. This could be because the timing is retarded to TDC.

I looked for a vacumn leak, did not find anything.
It runs ok with the timing bypass in test mode, so I feel the fuel injection is ok.
The spark pulse is steady thru the timing light with the bypass in test, but its erratic when the bypass is in normal state.
I think I will try to find a used computer, but I am starting to think that it's not this.

I may need to buzz out all the wires going to the computer, just in case one or two are broken. Otherwise maybe I should give up and scrap it.
 
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Old 12-04-10, 04:07 PM
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Last update.
Since this is a plow truck, it has 2 batteries. Battery positives are joined together, and each battery grounds to the engine.
While doing the compression check, I broke off what was left of a correded ground connection at the exhaust manifold. I replaced it with a new cable and carried on from there.
Turns out the new cable was not so good somewhere, when I used a jumper cable to ground that battery to the engine it ran fine. Idled well and tach worked.

Truck still has some power issues which I will sort out over time, but it runs and I can be ready for the snow.

So for anyone reading this post... if you have some wierd engine problems check your ground connections especially if you have 2 batteries.

I will upload a pic of what was my muffler after a pretty good back fire.

This also pulled the weld off the cat pipe leading to the muffler.
Thanks
 
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Old 12-04-10, 05:38 PM
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Hi TOHeating,

Give me some time on this and I'll get back to you. ASE MASTER is like my mentor. Rather than simply answering a question, he to had to write a novel. I need to read through everything you wrote and ASE's answers. Lord help me.
 
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