WOT Stumble

[slomove]

I was trying to drive out today and found after only a mile on the freeway that I have an intermittent stumble at 1/2 to full throttle. The AFR gage gets a bit erratic when that happens and after returning I found #1 plug a little black around the rim but not horribly so.

 

Any idea what to check first? Can it be the injectors?

 

Thanks!

[scannon]

Have you replaced the fuel filter lately? That's a symptom of one starting to get plugged up but it could be a lot of other things as well.

 

Bad plug wire

Injectors needing cleaning

Engine sensors

Loose electrical connector

 

I suspect you know all the possible failure points.

[slomove]

Thanks Skip, have not thought about the plug wires. Just to make sure the mechanics are O.K. I confirmed compression (190,200,190,200) and I blew through the injectors with pressurized brake cleaner. Of course I can not measure the flow but the spray pattern looked alright to me.

Then I replaced the ignition coil since it is easy to do and I still had one laying around. When it gets cooler I will do another test drive with data logging.

 

I replaced the pump intake filter maybe 2 years ago but I never replaced the high pressure side filter so if the problem is not gone, I will hook up a remote fuel pressure gage and check if that shows a drop when accelerating.

 

Really annoying, 2 weeks before the big trip.

[slomove]

In the meantime I checked the injection pressure, looks good but can not test anymore at high speed because now the car barely runs at all. One thing I noticed is the timing light is very erratic and the displayed rpm flips back and forth between 2x the actual rpm (normal for the wasted spark system) to 1x actual rpm. If my ECU is not broken (aargh!) then I guess it can only be the crank position sensor.

 

Any other ideas? I will swap the crank sensor tomorrow and if that does not help then I need to get a scope from work and try to get more into detail. The only funny thing about that is I was writing with Automoda who was wondering to come along on the tour but has almost the same symptoms disabling his car.

[scannon]

Injection pressure can be misleading if you cannot test it under load. It might be worth it to change your fuel filter(s). You might have got a tank of dirty gasoline. Can you easily look into the tank to see if there is any sediment on the bottom or floating around?

[slomove]

I could test it under load if the car was running well enough. I have a few feet extension hose to the gage. But since the car ran like crap now even slightly above idle I don't believe anymore it is the pressure (to be confirmed).

 

I did hook up the scope and the pulses look just fine, no gaps or doubling. Means the ECU and crank sensor are probably good. As a last resort I swapped the plugs. They looked just fine and the Iridium plugs should last forever, but with the (very much used) Bosch Platinum4 that I have in now, the engine idles smoothly and the erratic timing light is gone. Weird...can it be the plugs?

[scannon]

Does it drive OK with the Bosch plugs? I use NGK regular plugs in my turbo Miata engines with good life. Spark plug wires are a problem in the Miata engine, seldom lasting much more than 40k miles sometimes going bad in only 20K. When they go bad they have symptoms very similar to your car's,

[slomove]

Just tried it out on the freeway. Does not mean much at 6pm in Los Angeles traffic but so far so good. Maybe it was just a bad plug although I have my doubts. But lacking any more symptoms I have to be satisfied that the car runs.