Surging problem and cause

[kevinde1]

Hello guys I'm new to the forum but I've been visiting it regularly since I've bought this truck. I recently bought a 1987 SWB with 5.7l TBI. When I got the truck it had a surge that seemed to get better once it was warmed up. I thought no big deal its a good deal I'll figure it out (lol). Now for the meat and potatoes.
cleaned grounds
baseline timing @ 0 degrees this was not the case when I bought it.
new fuel pumps@14psi
New Filter
rebuilt throttle body

previous owner replaced all the sensors
EGR valve
TPS
IAC
CTS
knock sensor
map sensor
oxygen sensor
distributor
I have tested the TPS and MAP and distributor pick up coil and they are in range. EGR is clean and clear ports at the manifold. I'm pulling 21Hg in vacuum using a vacuum gage I checked for vacuum leaks all over the place and doesn't seem to be one. I've reset the IAC and reset the computer.
Here's what is happening when I put a hand held vacuum pump on the MAP sensor and put 21Hg at the sensor it kills the engine. If I use a can of carb cleaner I can keep it running. It starts to cut out at 17Hg and the MAP will cut the fuel off and kill the engine. So now I see why its surging the MAP reads 21Hg kills the engine, the engine starts to shut off then there's low vacuum at the MAP and then engine picks back up and starts the cycle all over again. If I put a hand help vacuum pump at the MAP and put 15Hg on it truck runs smooth but a little rich but increase that to 21Hg where it should be, at 17Hg it starts to die. Anybody know what would cause this? this leads me to the ignition control module or the ECM does anybody know how to test ICM? Anybody have any Ideas? Another thing I'm going to check today is the thermostat with the temperature gun its reading 145F at the neck of the radiator hose. Thank You for your responses

 

[kevinde1]

Here's an update changed thermostat to make sure I have a 195F. Fuel pump is wired in with a toggle switch to make sure ECM is not shutting off fuel pump. I have a code reader no codes are coming up. If hook up the MAP to the vacuum behind throttle body I can keep running if I work the throttle frequently but it will not hold a steady RPM and it just surges up and down at higher RMP.

 

[1987 GMC Jimmy]

Hello guys I'm new to the forum but I've been visiting it regularly since I've bought this truck. I recently bought a 1987 SWB with 5.7l TBI. When I got the truck it had a surge that seemed to get better once it was warmed up. I thought no big deal its a good deal I'll figure it out (lol). Now for the meat and potatoes.
cleaned grounds
baseline timing @ 0 degrees this was not the case when I bought it.
new fuel pumps@14psi
New Filter
rebuilt throttle body

previous owner replaced all the sensors
EGR valve
TPS
IAC
CTS
knock sensor
map sensor
oxygen sensor
distributor
I have tested the TPS and MAP and distributor pick up coil and they are in range. EGR is clean and clear ports at the manifold. I'm pulling 21Hg in vacuum using a vacuum gage I checked for vacuum leaks all over the place and doesn't seem to be one. I've reset the IAC and reset the computer.
Here's what is happening when I put a hand held vacuum pump on the MAP sensor and put 21Hg at the sensor it kills the engine. If I use a can of carb cleaner I can keep it running. It starts to cut out at 17Hg and the MAP will cut the fuel off and kill the engine. So now I see why its surging the MAP reads 21Hg kills the engine, the engine starts to shut off then there's low vacuum at the MAP and then engine picks back up and starts the cycle all over again. If I put a hand help vacuum pump at the MAP and put 15Hg on it truck runs smooth but a little rich but increase that to 21Hg where it should be, at 17Hg it starts to die. Anybody know what would cause this? this leads me to the ignition control module or the ECM does anybody know how to test ICM? Anybody have any Ideas? Another thing I'm going to check today is the thermostat with the temperature gun its reading 145F at the neck of the radiator hose. Thank You for your responses

Regardless of results, it sounds like you've got a good handle on the situation. Did you probe the three pins on the MAP sensor connector to see what was going on with them? If not, I think that should be your next step. Also, did you find the truck running better with the MAP sensor unplugged? The new sensor could be bad, there could be a wiring problem, or your ECM reference could be bad, likely condemning that guy. I'll attach a really nice set of MAP sensor tests that I found with pictures and thorough explanations. I think you'd be happier advancing that base timing a little. I know the sticker says 0*, but see about 8-10*. It'll feel peppier and make more out of the gas it's burning. Why do you suggest the ICM? I'd say the common manifestations of ignition control module problems on a TBI are either getting no spark, or getting spark and no injector pulse. If you want to, now or later, test it, you can remove it and take it to your local auto parts store. They'll put it on the bench and test it for you. The machine is not always right, but it's far more right than wrong. Also, before you start with the thermostat, see what the operating temp is on the side of the cylinder head. That'll give you a truer result, I think. Since you already changed it, it's good preventative maintenance.

http://easyautodiagnostics.com/gm/2.8L/map-sensor-multimeter-test-1

 

[kevinde1]

I have done the test on the MAP sensor and it checks out perfectly. My thought about the ICM is because what is essentially killing my motor is the injector pulse shuts off. When you apply 20hg to the map sensor which is exactly what the engine is pulling (19-21hg) it starts killing the injectors at about 16hg. At 20hg my MAP is reading 1.1 right where it's supposed to be. So my thought was the ICM is killing the injectors when there is 20hg applied to the MAP. The ICM is new along with the distributor. I can tell previous owner has been chasing this problem due to all the new parts on the vehicle.

 

[1987 GMC Jimmy]

Did you try test 2 and 3 on that link? I understand you're getting 1.1V at 20 inches of Hg, which points to a good MAP sensor and a good ECM reference, but I don't think it necessarily answers a 12V wire or ground wire problem. I don't know if the ICM can work or not work in such a complex way. When people have these problems, there's not even an injector pulse at startup. It doesn't fade out like that; it just doesn't work at all. Of course, there's one way to be sure, and that's taking it out and having it tested. Make sure they test it three or four times. If the current ICM tests okay, make sure you get some of the white grease to put on the bottom of it. When you have a complicated and drawn out problem such as this one on a TBI vehicle, it'd be a great idea to make a cheap scan tool and get live data from the truck as it runs. Here's how:

http://www.gmsquarebody.com/forum/showthread.php?t=13194

 

[kevinde1]

yes I did test 2 and 3 and they both check out. Thanks for your replies

 

[1987 GMC Jimmy]

Okay. I'm sorry to have kept nagging about those. I hope someone else chimes in.

 

[kevinde1]

Well I pulled the ignition control module went to the local parts store picked up a cheap value craft one that's all they had, compared some ohm reading on the 2 and there was a lot of difference between the 2. installed the value craft one and I'm getting spark but now no injector pulse at all. LOL figures right.
I also pulled the ECM opened it up and it has all DELCO parts in it and is very clean inside and out for a 30 year computer. Still getting no codes even with no injector pulse with value craft ICM.

 

[chengny]

Here's what is happening when I put a hand held vacuum pump on the MAP sensor and put 21Hg at the sensor it kills the engine. If I use a can of carb cleaner I can keep it running. It starts to cut out at 17Hg and the MAP will cut the fuel off and kill the engine. Anybody know what would cause this? this leads me to the ignition control module or the ECM does anybody know how to test ICM?

At first I was wondering how you determined that the reason for the final drop in RPM (to the stall point) is because the ECM has commanded the injectors to stop opening. But then I see where you can keep it running with an external fuel supply (the carb cleaner) and that you also have the FPR by-passed to eliminate the control circuit as a possibility.

Okay, you have fuel pressure always available at the injectors - independent of ECM control, have observed that the engine will continue to idle on ether.

I'm sure that I am preaching to the choir when I write the following - but just to keep things right in my own head:

The ECM controls fuel delivery based primarily on two inputs - manifold pressure and engine speed.

The manifold pressure signal is supplied by the MAP sensor. The sensor is basically just a pressure transducer, which generates a voltage that corresponds to the absolute pressure in the manifold. It is supplied with 5VDC from the ECM and returns a 1 - 4 VDC signal to it.

Your MAP sensor has been replaced once and also checked for proper calibration throughout the full range.

Tests have shown that engine will hold a steady, continuous idle with the MAP sensing line disconnected. But when the MAP sensor is connected to normal idle manifold pressure of about 21 in Hg (either from the manifold or a test pump), fuel injection is promptly cut off.

In one way, those facts would appear to implicate the MAP signal circuit. But on the other hand, the sensor has been replaced and checked. Not only that; but I don't believe that - while idling - the engine management system, even if it receives the lowest/zero voltage input signal from the MAP sensor will completely shut down fuel injection.

AFAIK - Complete shutdown of fuel injection occurs only under 3 circumstances (other than when the engine is purposely turned off):

1. Periods of extremely rapid deceleration from high speeds - and then only for
a short time. An emissions thing - to allow any fuel remaining in the
manifold to be burned off. This is the only condition that is related to MAP.

2. If engine speed exceeds the point that internal damage could occur - IDK
the RPM. In this case, fuel delivery resumes when RPM's fall back into safe
range.

3. If the ECM loses the pulsed reference signal from the distributor (i.e. the
ignition control module)

So, having said all that, I have to agree with you. Taken together, all of the above would seem to suggest that the ECM is shutting down fuel injection due to the loss of a pulsed signal from the distributor ignition control module.

A couple of questions:

1. When you set the base timing to 0 BTDC, you did it with the TAN/BLK (timing set) lead disconnected - right?

2. You checked the P/N switch for proper operation?

3. Same for the A/C on switch?

The P/N and A/C switches shouldn't be the issue. These are intended to provide the ECM with a warning of an upcoming load increase (when the contacts are closed) and that it should increase idle speed - not drag it down.

I don't know enough about ignition control modules to say whether or not one would work fine at higher RPM's but then fail to deliver a reference signal as engine speed drops. Below is the procedure for testing the ignition system - including the ICM specifically in step 3. It is written for a non-start condition so it doesn't exactly apply to your situation, but it might shed some light:

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[chengny]

Still getting no codes even with no injector pulse with value craft ICM.

Yeah I was going to ask if you have ever seen an SES warning light - but I forgot.

Check it by turning the ignition to RUN. The SES lamp should illuminate. Start the engine and it should go out. Here is the whole diagnostic procedure:

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[kevinde1]

The P/N switch works like it should as far as the A/C switch I'm not sure how to check that. A/C doesn't work, it's there maybe low on Freon, guessing that's not what you mean by checking the switch. Thanks for all the good input I'm gonna call it a night for now and try to get my hands on a good ICM tomorrow and give it a try.

 

[chengny]

The P/N switch works like it should as far as the A/C switch I'm not sure how to check that. A/C doesn't work, it's there maybe low on Freon, guessing that's not what you mean by checking the switch. Thanks for all the good input I'm gonna call it a night for now and try to get my hands on a good ICM tomorrow and give it a try.

I wouldn't worry about it - like I said, that switch will only serve to increase the RPM. It comes off the lead that runs from the control switch (on the accumulator) over to the compressor clutch:

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If you want to be sure, just pull the connector off the switch.

 

[kevinde1]

Well I took the value craft module out and put old one back in, got the truck started again and of course same problem (no injector pulse at idle). I ordered an AC Delco ICM on Ebay for 25 bucks and it will be here in 2 days so in the mean time I'm going to install some 2" drop shackles. I will report back with the results after the new module is installed.

 

[1987 GMC Jimmy]

What brand was the one that was in there when you first got it?

 

[chengny]

And the distributor is new?:

previous owner replaced all the sensors
EGR valve
TPS
IAC
CTS
knock sensor
map sensor
oxygen sensor
distributor