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-   -   Intermittent crank-no-CEL-no-start when engine warm - Bad ECU? (https://www.mr2oc.com/2-mki-84-89-na-sc/679404-intermittent-crank-no-cel-no-start-when-engine-warm-bad-ecu.html)

samglad June 20th, 2019 12:31

Intermittent crank-no-CEL-no-start when engine warm - Bad ECU?
 
My bone-stock 88 na likes to crank-no-start roughly 50% of the time I try to start her up after she's reached operating temperature. When this happens, my CEL does not come on with the key in the on position (or while cranking). After under an hour, the problem goes away on its own.

I finally caught it at a convenient time yesterday, and following the BGB's process in FI-28, I saw that the CEL did come on when I grounded ECU terminal W to the body. I tried the next step of looking for a voltage drop between ECU terminal E1 and body ground, but I didn't have any real luck there. Is there a diagram that shows exactly where on the body E1 ultimately grounds?

Here's where it gets fun though. With the key in the on position the whole time, I went round-robin testing voltage between ECU pins E01/E02/E1 and BATT/+B1/+B (all using T pins to back-probe with all connectors plugged in), and the lowest voltage I found in any of those combinations was 11.9v. Correct me if I'm wrong but this seems (?) to suggest that the problem is not voltage drop somewhere between the battery/EFI relay/fuses/body ground and ECU that is outside of the ECU... right?

As I went to double-check battery voltage after all that, I heard a little almost-hiss sound in the engine bay, and sure enough, my CEL had come back on and she fired right up when I turned the key. So... am I being lazy for wanting to try another ECU, have I misunderstood my own voltage tests across the various power/ground pins on the ECU, or is there something else I've overlooked? Should I try jumping the fuel pump via the service connector next time this happens, or is there no point? Huge thanks in advance for any feedback.

mikemo June 21st, 2019 06:42

Samglad,
I have a suggestion. You checked the voltage on the ECU and the coil with the key on, but did you check it while cranking? I had a problem once where I had no-start and no-spark, but saw +12 volts on both the ECU and my coil. After some more frustrating troubleshooting I measured the voltage while cranking. As soon as the engine started to crank, the voltage on the ECU and coil dropped to zero. Another clue that I missed while originally troubleshooting was that my CEL would go off as soon as I started cranking.

I replace the EFI relay with a spare and it has worked ever since. As soon as the fuel pump starts, the current draw through the relay increases dramatically. In my case that was causing the marginal contacts to open up and voltage drops to zero.

Good luck, and let us know what you find.

rmeller June 21st, 2019 14:49

You should get CEL with the ignition on, and if you don't, either the ECU is bad or is not powered up, or the lamp is bad. You tested the lamp. The intermittency of the problem suggests a bad connection, and that can hide many places. The connectors themselves are more likely than internal to the ECU. Inspect everything that you can get at, looking for corrosion, heat damage, or mechanical damage. Keep repeating your tests. When you have an intermittent problem, single tests can be misleading.

I think that all of the control system grounds go to a screw on the intake manifold.

samglad July 2nd, 2019 15:58

Apologies for the delay here; life got in the way a bit, but thanks for the replies.

Quote:

Originally Posted by mikemo (Post 6801284)
Another clue that I missed while originally troubleshooting was that my CEL would go off as soon as I started cranking.

I replace the EFI relay with a spare and it has worked ever since. As soon as the fuel pump starts, the current draw through the relay increases dramatically. In my case that was causing the marginal contacts to open up and voltage drops to zero.

Thanks for the suggestion; I'll definitely see if I can get another pair of hands to try this out. The big difference in my case is that when this happens, the CEL is always off with the key in the on or start position. She's never died on me while running, but I've seen it happen multiple times immediately upon turning it off. I actually did replace the EFI relay just to see what would happen a while back, but no change, sadly.

Quote:

Originally Posted by rmeller (Post 6801294)
You should get CEL with the ignition on, and if you don't, either the ECU is bad or is not powered up, or the lamp is bad. You tested the lamp. The intermittency of the problem suggests a bad connection, and that can hide many places. The connectors themselves are more likely than internal to the ECU. Inspect everything that you can get at, looking for corrosion, heat damage, or mechanical damage.

Thanks, rmeller. I had this happen again this past weekend - but for the first time, while the engine was cold, and only for a few minutes - and went through everything again. Here's what keeps making me wonder about the ECU: the worst voltage drop I can find while backprobing the ECU connections with T-pins while the issue is occurring is 0.3v. Meanwhile, the voltage at each pair of +/- pins at the ECU doesn't seem to change between when the issue is occurring and when it isn't. The only thing that keeps occurring to me is that maybe I've got a relay that isn't behaving when it's hot - and they all were fairly toasty to the touch when I was troubleshooting the other day - but only after it opens up due to killing the engine... But if it was a relay, wouldn't I fail to see near-battery voltage at some pair of ECU connections or other?

While I'm not one to just throw an ECU at an electrical issue, does this seem fishy, or am I just testing like a meathead? As a bonus, I haven't been able to find any body grounds with a nontrivial voltage drop, including the one on the intake manifold (thanks for that tip, by the way). I'm tempted to open the ECU up and check for leaky capacitors, but again, if that's overkill, I'm open to that feedback.

Thanks again for the suggestions on this.

mikemo July 3rd, 2019 07:09

Samglad,
When you grounded terminal W on the ECU, all that does is prove that there is power to the CEL circuit. The CEL would come on even if you didn't have the ECU connected.

Since you were measuring on the connector pins, perhaps there is a problem with a dirty or loose pin in the connector?

I've never been inside an ECU, but perhaps you can take the cover off and probe the pins on the ECU circuit board side to see if your power/ground is good at the ECU's circuit itself.

If you have close to 12V on all the pins that you say you tested in your first post, I'm starting to agree that your issue might be the ECU.

Good luck.

bktheking July 10th, 2019 10:51

I have been inside mine, soldering looks good for the most part but you may want to look at the solder joints around the to220 transistors for cold solder joints and inspect that lytic caps for leaking/bulging, IIRC there are 5 or 6 of them.


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