Hello Forum members,

While I've been lurking for awhile, I could use your help with an odd problem.

My Blazer's an '87 K5 full size with a 5.7L TBI engine, auto trans, and stock EXCEPT for an aftermarket tachometer, aftermarket transmission cooler, and 6" lift w/35" tires.

History:

My 87 K5's A/C compressor failed last spring, spraying oil under the hood. After replacing it, everything's working fine. Last fall, I washed the with truck with a garden hose prior to winter setting in. This included the underside/frame as well as under the hood; removing the oil stain from the compressor failure. I covered the distributor and coil prior to washing the underside of the hood. I did not spray water directly on the rear of the engine; only at the front of the engine (water pump) and the underside of the hood to remove the oil from the compressor failure.

After washing the truck, it immediately developed a misfire under load above 2000 RPM's. I thought something under the hood had gotten wet, so I waited a few days for everything to dry out. I took the truck out on the highway, and the misfire was still present. Under load, the engine would begin to cut out/lose RPM's above ~2000 RPM's. The tach would be steadily climbing, and abruptly lose 200-1000 RPM's over a second or two; as if the engine suddenly died. But then, the engine would fire up, backfire out the exhaust, and resume normal operation. And stall again. And again. Then, without me doing anything, the truck would run fine. For miles. And days. Then suddenly, the misfire would return. And would happen for a day or two. Then, the problem would go away again for days. I didn't have time to deal with the problem, so I did nothing about it.

Winter has set in. The problem of the on again-off again misfire is still present.

Symptoms:

While the on again/off again misfire is still present, other electrical problems have become evident.

Sometimes, when using the turn signals (left or right), 4-way hazards, or brake lights, the engine abruptly stalls; immediately to 0 RPM's. It doesn't matter if the engine is at idle, or driving down the road at 2000 RPM's. When this "turn signal/hazards/brake light" problem occurs, the engine immediately stalls (0 RPM's) and the SERVICE ENGINE SOON (SES) lamp blinks on and off in alternate unison with the turn signals. What's happening is:

Right turn signal (and green arrow on the dash) turned on, engine immediately stalls/shuts down (0 RPM's).
Right turn signal arrow ON, SES off.
Right turn signal arrow off, SES ON.
Right turn signal arrow ON, SES off.
Right turn signal arrow off, SES ON.
And so on until I turn off the turn signal by centering the turn signal/wiper lever on the collum. Once centered, the SES light stays ON steady. I then turn the key OFF. Then restart the engine. The engine immediately starts, but idles at ~1500 RPM's for a minute or longer after restarting. After this brief period of time, the RPM's return to normal (~750).

I borrowed a friend's scan tool and observed the following condition:

No codes stored.

All engine functions/sensor readings are normal.

With engine at idle, I step on the brake pedal and leave it depressed. Engine stalls, no SES light. Data on the scanner will disappear; as if the scanner has been disconnected from the ALDL port. Once I remove my foot from the brake pedal, the SES light will come ON, and the scanner will then display data from the ECM. All data appears normal, including 0 RPM's (engine stalled). No codes are present.

The same thing will happen when using the turn signals or 4-way hazard flashers.

Engine at idle, turn signal or hazards ON, SES light will flash in alternate unison with the green arrows on the dash, and the data will disappear from the scanner (as if unplugged from the port). Turn off the turn signal/hazard, the data will reappear. No codes/anomolies are revealed.

An additional symptom was also noticed while trying to diagnose this problem:

Under full / WOT acceleration, the SES light will begin to steadily brighten. As the RPM's increase, so does the intensity of the SES light. The SES light will become noticeable around 2500 RPM's, and get brighter as the RPM's increase. At ~5000 RPM's, the SES light is bright enough to notice during daylight hours. (I discovered this while driving at night with my dash lights off).

What I've done to diagnose the problem, without success:

Inspected and cleaned all ground wires under the hood.

Removed the applicable fuses for the exterior lamps. With fuses removed (no working external lamps), problem doesn't occur. Once fuses re-installed, problem returns.

Removed the flasher/hazard units. Problem doesn't occur. Once re-installed, problem occurs.

Removed the applicable bulbs; front, rear, and side, of the external lamps. Problem occurs.

Removed the "green arrow" bulbs from the dash. Problem occurs.

I have NOT replaced any components/parts/sensors/ yet.

I have NOT been able to find/inspect the ECM ground.

While I consider myself a reasonable weekend mechanic, this problem has me stumped and frustrated. The key concern for me is the fact the ECM seems to lose it's power supply/ground during this condition. The common "trigger" for this problem seems to be the use of the exterior lamps. Keep in mind, I'm not blowing any fuses at any time. Because this problem occurs randomly, I've had a hard time chasing it down. The truck will run fine Monday to Thursday, then misfire and stall on Friday, then run fine on Saturday and Sunday.

The only place I can see anything in common with the ECM and exterior lamps is the:

Steering collum (tilt).
Wiring harness running the length of the left side frame.

However, this may not explain the misfiring above 2000 RPM's.

In summary, my truck does the following things randomly:

Misfires above 2000 RPM's.
Stalls when using brake lights/turn signals/hazards.
SES light gets brighter as engine RPM's increase.


So, after my first rather lengthy post here at The Blazer Forum, can anyone offer any suggestions on how to correctly identify and repair my K5's problem(s)? I enjoy driving it very much, but this problem has kept me in the driveway for a while now. I simply don't know where to look for this gremlin.

Thank you very much in advance.

Sincerely,

Rob

 

The only thing i can think if is a bad alt or maybe bad grounds.

 

Hi Irv Jr,

I also considered the alternator as an initial suspect; since I did wash the engine compartment. However, it has since tested fine both on a bench and on the engine. If something is wrong with it, it's intermitten and hiding when it's placed under an electrical load.

The only ground I haven't looked at is the ECM; I can't seem to find it's location on my '87.

PS: The truck ran fine over the last two days. Back at square one...

 

The ecm should be behind the glove box under the dash. Also check your engine ground, should go from the back of the head to the firewall. I have seen them cause stupid stuff if missing or broken.

 

Hello Mighty Mouse,

I see the ECM behind the glove box; but haven't been able to find any grounds up in there. The only place I saw a ground-to-chassis lug was behind the dash; beside the lower left air duct (above the parking brake) and that connection was good. Any suggestion where the ECM's ground may be located?

I've also cleaned all the engine grounds; including the firewall-to-cylinder head. In addition, I ran separate 12ga grounds from the firewall and frame to the negative side of the battery to eliminate the possibility of a bad/intermitten connection. No luck either; problem is still present.

FYI: I clean grounds by removing the bolts, cleaning the threads on the bolts and bosses, running a wire wheel/brush on the surfaces, reinstalling the bolts with brand new star washers, then checking for circuit with my multimeter (0.01 ohms resistance).

 

The ECM just mounts there and there is no wire grounding it at the mount.

 

OK ... the ECM's mount is the ground; gotcha!

I was looking for a black/white stripe wire going to a ground lug ... DOH!!!

I'll check it out and report back. Thanks!

 

Update to all concerned:

After many frustrating months of trying to diagnose the problem, at last ... success.

Replaced ECM. Problem solved. Truck runs great.

I'm at a loss to explain how the various symptoms related to the original ECM, but the bottom line is the ECM was the sole culprit. No other parts have been replaced nor repaired.

My original post was made a long time ago, I felt I should follow up with outcome. This was a very difficult problem to correct. My truck would run great for days and weeks at a time, then suddenly act up. I would limp home, pop the hood, attempt to troubleshoot, then like magic ... the truck would run fine without ever touching anything. I avoided "throwing parts" at the problem to make it go away; hence my long term attempt to correctly troubleshoot the defective component.

The "eureka" moment came when the truck suffered an "episode" and threw one of it's tantrums. As noted in my original post, the engine would "high idle" ~1500 RPM's for no apparent reason after one of the episodes. I captured a fault code of "XX" on the scan tool once the engine restarted. Then immediately thereafter, no codes were stored. Once the ECM was replaced, the truck has run like new ever since.

Thanks to all that have taken the time to read and consider my problem.

Sincerely,

Rob

 

Thanks for posting back. Many times people suspect PCM problems when it''s something else, often a ground. This is one of a minority cases where the PCM actually was the problem, when it looked like it was bad ground or power.