Intermittent CEL at low RPMS

woodiwoodi Wagonist
in Electronics, Fueling & Air Conditioning
Just wondering if anyone has any ideas on this one. I have a dohc zc in my wagon. Running a PG7 ecu. The engine runs fine but when running around 2nd or 3rd gear cruising through town around 20-30mph the cel will comes on. It seems to be mainly when the engine is around 2-3k just cruising. It will usually go off within about 30 seconds when I accelerate out of town or stop. I have checked my ecu for codes both parked and while driving down the road with the light on and I never register any codes. All my searching and reading on the topic has pretty much pointed me in the direction of my ecu being bad. But due to the consistency of the CEL always coming on at the same time I was thinking maybe it actually is a legitimate issue that is registering but for some reason not storing a code.

I've checked my hazard fuse and it is good so It's not just automatically clearing because the fuse is blown. I do have another ecu to try which is probably the next step though it's a pm6 so it should run the engine but not as well and would not be a direct comparison against the the pg7.

Also i'm not sure if this could be at all related as well but I cannot quite get my ignition timing correct. I have a cheap ebay distributor on there and set it with the yellow plug jumpered and with the dizzy all the way to the firewall it is slightly retarded. I've run through just about everything else on that issue so I was assuming it was just a really badly made dizzy which I've been meaning to swap out to make sure. Could a bad ecu be pushing my timing out of whack though too? Maybe the cel is the o2 sensor telling me I'm running rich because my ignition is retarded and I'm getting a dirty burn at those engine speeds? Any ideas.

Comments

  • superhatchsuperhatch Moderator
    My thought was 02 sensor. I think if your ECU would be bad you would have other troubles. I had a bad ECU and the car wouldn't start at all, and I was getting a constant engine light (obd1 w/ cel switch).

    Do you only get it in 2nd and 3rd? Can you replicate it in any other gears?
  • woodiwoodi Wagonist
    I tried on my drive home today and couldn't do it in fourth or fifth It loves to do it in first and second going down my fairly long driveway and 2nd and 3rd through town. I changed the ecu out for the pm6 started the car and went to go for a test drive and bam cel the exact same way going down my driveway. Then to a stop at the end and it turns off. Also checked timing with the other ECU and it was exactly the same as it was with the PG7. I'm highly doubting I have 2 bad ecu's so it's gotta be something else. The timing issue makes me think something is definitely fishy but I'd hate to run out and buy another new distributor if thats not the root of the problem.
  • woodiwoodi Wagonist
    So just to keep this up to date in case some one reads this and has any ideas. I drove about halfway to work today on the pm6 (about 12 miles) and it ran fine but had a bouncing idle at stop signs (1k-1.5k) as well as the rpm's holding on when I let off the clutch. Switched in the PG7 and it idled normally and might hang a little bit but not anywheres near as much when shifting. I'm thinkg those problems would probably be fixed by adjusting the idle correctly for the pm6 ecu. No CEL the whole way to work but that's pretty much all 60mph driving so it never really comes on during that trip. But it definitely seems to me that both computers are fine so it has to be something else causing it.
  • bikewrenchbikewrench Wagonist
    Have you made sure your mechanical timing is correct?
  • woodiwoodi Wagonist
    yup it is dead on... up arrows up lines on cam gears in center horizontal 6mm punches in the cam shaft holes. Single line on pulley is lined up with the pointers on the valve cover. I've had my timing belt on and off a couple times in the process of trying to get this engine running right.
  • woodiwoodi Wagonist
    So I was able to narrow it down to a fairly exact set of conditions to get the cel to come on today. I even was able to get it to happen in all the gears 1-5 if I followed the same process. The trick is to shift into your desired gear then drop the rpm's to 2000 or slightly below then give it a little gas to maintain your speed and bam the light will come on. After that it pretty much stays on until RPM's drop below about 1500 at which point it shuts off until the 2k little bit of gas situation happens again.

    I'm thinking maybe since the engine is bogging a little trying to give the wheels a little power at 2k rpm's combined with my timing being a little retarded it's running just rich enough at that point to have the ecu throw a code. Not really sure if that even makes any sense just an idea I came up with. The whole thing seems so weird.
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