5:30 AM Fuel Injection Mystery

[DogRocker]

I'm stumped. 1981 FI. I take the car to work on nice days. Lately that's been often.

Every few days...maybe every 5 - 10 days. It won't stay running first thing in the morning. All other mornings and when I leave work in the afternoon, it starts right up and the idle settles somewhere between 900 - 1100. Rev it a little and idle settles again.

Then occasionally, like this morning, it'll start but it runs rough almost like all the cylinders aren't firing and the idle sags until the engine just dies. If I try to rev it to keep it alive it hesitates, and I can kinda sorta keep it running but as soon as my foot comes off the idle drops and it dies after a few seconds. I note how much dew is on the windshield when this happens (to crudely gauge the humidity). No difference if it's a warm dry morning or cool slightly damp.

When this happens I take the truck to work. I come home in the afternoon and it's fine. Starts right up. Runs normal. Idles normal. No issues. Every time. The next day or six it's fine.

Cap & Rotor are new.
Plug wires are new.
Fuel Filter is new.

I'm stumped.

Any suggestions? Ideas? Voodoo rituals?

[aj81spider]

Check the air hose from the air filter to the intake (giant rubber hose over the top of the engine). I had a similar problem where the engine would occaisionally just lose a lot of power - then it would be fine. Turned out I had a crack in the hose and under certain conditions (say in a cool morning) the crack would open up and let air in. Other times it would be closed and everything worked fine.

It may not be that, but it's easy to check.

[GeorgeT]

You need to get the Bosch Fuel Injection troubleshooting guide. It may be something as simple as a coolant temperature sensor or cold start injector or auxiliary air valve.

[RRoller123]

The AFM flap may be sticking. Easy to check. With the engine OFF, pull off the big hose between AFM and Intake Plenum and see if there is any resistance as you manually move the door flap back and forth over its range of motion. There should be no contact or rubbing on the housing, but there frequently is. It can be resolved by sanding the housing with 220 or lighter grit. Rub marks will show where it is coming in contact with the housing. Hold the AFM unit in its vertical operating position when checking all this, as the weight of the door, if there is slight hinge play, will vary the result.

[DogRocker]

Check the air hose from the air filter to the intake.

I'll take it off, put on my magnifiers, twist-turn & inspect closely. Thanks.

You need to get the Bosch Fuel Injection troubleshooting guide.

I have B. Artigue's guide + plus another more general Bosch manual. I've gone thru the various suggested measurements and nothing stood out. I'll need to repeat when things are acting up.

The AFM flap may be sticking.

On one occasion, while it was acting up, I tried to drive it and got about two blocks from the house before it completely died and wouldn't start again. Walked back to the house, searched the message board for clues, grabbed my toolbox and walked back to the car. It seemed like it wasn't getting fuel. I left the key in the on position, took off the big hose and stuck my hand in to check the AFM flap. When I pulled my hand out, I thought I heard the fuel pump kick on. It started and ran fine. I went home. Removed the AFM assy., sprayed some lube toward the hinge, lightly sanded some of the flap edge, checked movement multiple times and reassembled. Still ran fine. Touching the AFM flap may or may not have had anything to do with solving this problem temporarily. That is, it may have started running simply because it always returns to normal operation after some period of time.

A few days later at 5:30 (ish) AM the issue returned. This time I went straight for the flap thinking it might be stuck. Nope, seemed fine. Still didn't start after a jiggle or two. Went to work with another car, came home, it starts and runs fine.

Fuel injector or two have become intermittent?
Fuel Rail Pressure Regulator?
Bad ground for fuel pump (grounded @ taillight?)?
Bad connections @ fuel pump?
Bad ground for fuel injectors (two ground wires @ valve cover?)?

I read somewhere here that 80% (or more) of FI issues are due to air leaks. Perhaps spraying carb cleaner around seals while it's acting up would reveal problem.

I'll get some new batteries in my headlamp to be ready for the next time.

[joelittel]

I had similar symptoms. Cleaning and resto-moding the ground pod under the dash, near the ignition switch, seemed to have solved my intermittent starting/stalling issues.

[spider2081]

The FI system is comprised of fuel, air, and electrical systems. You might find it helpful to keep a log of symptoms and steps you have taken to correct the problem. One important aspect of troubleshooting is to try and keep an open mind. There can be more than one issue causing what appears to be a single fault?problem.
1. do you have spark when the car will not run?
2. do you have fuel pressure?
3. does the car crank faster when it will not start
4 when removing the gas cap is there pressure of vacuum hissing noise?

[DogRocker]

Thanks for suggestions. I'll report back when I get somewhere.

[DogRocker]

I tried switching to Premium gas and since doing so have not seen the problem come back.