Tech Question - how to fix a strange miss-studder

#1

Here’s a strange tech question. At Road Atlanta, I was running essentially a new car. It started and ran fine. On the out lap of the Friday race, when the car was pretty hot, when I got to turn 5, I started to accelerate in 3rd gear at around 2000 rpm and the car started sputtering and missing badly. It did this for 30 or more seconds. I finally got frustrated and just started slamming on and off the gas, and eventually, the car pulled through it and cleared out. It tried doing it again on Saturday, briefly, during a red flag. It didn’t seem to ever have a problem if I was at full throttle in a racing RPM range, but these problems wind up showing up at a bad time. Any thoughts what might cause this?

Thanks in advance.

#2

Intake manifold gaskets.

#3

I’ve had occasional situations like this that threw a check engine light for the AFM. They haven’t returned since I replaced it with a rebuilt unit, but intermittent problems have a way of making you believe you solved the problem and then returning again. Is your AFM original?

#4

Engine management problems can be a bastard. While fighting this kind of problem in 2009 or 2010 I installed a fuel pressure gauge and a F/A gauge on my dash. Then I connected the latter to the data logger. This helped me a lot to rule out possible causes. The symptom was frequent severe missing at high rpm at WOT. Not always, but frequent. Ultimately the cause ID’d as the AFM. There’s a school of thought that you can richen/lean the mixture by loosening/tightening the AFM door spring. Some well-meaning types did that to my car. It’s a bad idea.

I say “ultimately the cause was ID’d…”, but I went thru hell trying to ID the cause. For example,I thought that I found evidence of a little voltage drop across some key points. A couple things I did was disassemble every ground that, by any stretch of the imagination, could have something to do with the engine. I abraded the surfaces and reassembled the grounds using conductive grease. I also cut out the round connector that goes to the injectors and created individual connections, abraded and greased, for the injectors and temp sensor. That was probably all a wild goose chase.

Re. CEL. I get CELs for odd reasons every once and now. I used to take them seriously and tried hard to fix them. Now, as long as the engine seems to be running ok, I ignore them. If you’ve got a real symptom and also a CEL, it’s worth doing the stomp test to see what info you might get. But if your engine is running fine and you get a CEL, screw it.

#5

Stomp Test? Nah, I’ve got an '87 with Motronic 1.1, which has four error codes. I get O2 sensor codes from time to time, which I used to care about.

After the worst bit of stuttering the car ever had, it threw codes 1 and 2, for AFM and 02 sensor.

#6

Get a later DME like a 173. That said, I don’t recall that I’ve ever gotten anything useful out of the stomp codes.

#7

We changed the AFM this weekend, it helped a bit but not all the way. Still a miss on the out lap, but nothing under full throttle. The intake boot may have a small tear in it, we siliconed it for the weekend, I dont think its leaking, and even if so, it wouldnt seem too be leaking enough for a miss. Is it possible that a vaccumn leak is doing this?

#8

Did you replace the intake manifold gaskets? That should be your next step. Think of it like this: a vacuum leak before the throttle body is like a hole in a kidde pool. After the throttle body it’s like waterboarding. It totally drowns the engine in air and ruins the fuel air ratio.

#9

Is there anywhere that sells all the upper intake hoses and the like in one package? These cars have a pile of air hoses on the top side of the engine. I might change all of them plus the intake gaskets on this car.

#10

O2 sensor might be worth a try if you have a spare. I had a friend that said he had similar issues that went away at WOT. I think that is a different air/fuel map that doesn’t use O2 sensor data (I could be wrong, happens often).

Easy to change and one less variable. Don’t do it when you change all of the hoses, then you won’t know what fixed it:)

#11

To bring this thread to an end, the car had a loose fit from a spark plug wire to the cap, and it had eaten up the connector. A new cap and wires fixed it nicely.