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Friend's FD stumbles and stalls at half/full throttle

Old 04-27-19, 11:22 AM
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Friend's FD stumbles and stalls at half/full throttle

Alright, time for you guys to lend a hand .

Working on a friend's FD. 93 touring, almost stock (just downpipe and cat-back - stock ECU, intake, IC, main cat, everything else).

Car starts and idles fine. Engine has weak-ish compression (original motor, 73k miles) but has a steady idle.

Even just sitting in the driveway free-revving the engine it will stall, pop, and break up when you give it decent throttle. Easing into the throttle you can only get to about 4500 RPM and it won't go any higher. If you ease into the throttle and go full throttle it will sputter then stall out, engine will full on stall and die. Starts right back up after this, though. Same deal driving around - give it some throttle and it starts popping, jerking, or stalling.

Things I've done so far -

- New leading plugs (BUR7EQP), trailing plugs were OK
- New coil pack harness
- Tested stock coils, they passed shop manual test
- Tested fuel pressure - pressure stays around 38 psi, if I yank the throttle open and get it to stall it stays solid, no movement on the fuel pressure.
- Front secondary injector was dead - clicking with 12v/ground got nothing, no resistance. Replaced it with good used injector (really thought this was it!)
- Rear secondary injector clicks with 12v, forgot to test resistance.
- Swapped TPS out with a spare, don't know if the spare is 100% or not but it's doing the same thing with the spare

ECU is throwing no codes at all.

Wiring harness on the engine is in decent shape, it's not crispy or funky. Some vacuum lines have been replaced with silicone (by me years ago) and it has my check valves.

I'm leaning towards something with the secondary injectors. Not sure if it could be the injector driver in the ECU, that's a possibility.

Any good ideas? Anyone seen this before?

Thanks!
Dale
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Old 04-27-19, 11:34 AM
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So I may have just fixed it

Found a (really ratty looking) ECU in the attic and swapped it in. Free revving the car it revs up no problems, all the way to redline.

Fixing to open the ECU up and see what's inside!

Dale
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Old 04-27-19, 02:23 PM
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Good!
Because I was thinking if Dale doesn't know what's wrong with it, what are the rest of us mere mortals going to do?
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Old 04-28-19, 09:08 AM
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I'm not perfect, I know a whole lot, but I learned it all from the community over quite some time. This one was stumping me at first!

To be honest I think I got lucky.

So, I pulled the passenger kick panel to get to the ECU originally to start testing inputs and outputs on the ECU. Having the thought that maybe an injector driver was bad, I figured it might be worth swapping in another ECU. Fortunately I had 2 to pick from - my original 94 ECU and a JDM 93 ECU. Grabbed the JDM ECU and plugged it in, Car fired up and settled to an idle, started revving it and I was shocked that it was revving freely and properly.

I opened up the ECU that was in the car - it is a 93 ECU (N3A1) but had the "remanufactured" on it. I know Mazda back in the day had some funky ECU issues on 93s and they had a TSB where they were swapping them out for certain problems. Opening it up and looking around I could see no damage - no leaky or blown caps, no damaged transistors, nothing obvious.

I also plugged my 94 ECU in for giggles and grins - car ran fine with it, save for a check engine light. The 94s all had the EGR with the switch so it threw a code since it didn't see that switch in the circuit. Good thing is that proved the check engine light was working like it should.

So, to make sure of everything, I swapped the original ECU back in the car. To my amazement, car fired up and ran fine. Huh? Revved up like normal. Drove the car around, drove great, made great power to redline, just 100%.

Either the ECU was stuck somehow - unplugging it reset something - or there was a bad connection on one of the ECU's pins that unplugging and re-plugging fixed. The battery had been disconnected a few times since this started happening so I don't think it was just simply needing a power reset.

FYI, I was able to replace the front secondary injector without having to do too much removal in the rat's nest. Removing the stock waste gate control/precontrol solenoids gave enough room to get to it. It's still a pain to get the injector out, a little PB Blaster and rotating and prying the injector finally got it popped out. The rear secondary probably would have required the air control valve to be removed.

Things to learn from this -

- When dealing with a general engine running issue like this, it most likely won't be a line in the rat's nest or a solenoid. People get obsessed with the rat's nest, a problem like this is typically more a general issue with the fuel injection system, spark, etc.

- If you're not sure if the engine is OK, stop what you are doing and do a compression test. Had a friend that spend AGES troubleshooting what seemed like a weird misfire. I took a look at it and found one compression pulse low on the engine due to a totally stuck side seal that was blowing compression into the oil pan.

- It's always worth putting in a fresh set of plugs. Rotaries are hard on plugs and I've seen a number of engines that had the plugs neglected for a long time. Plugs are cheap.

- A fuel pressure test can be a simple way to eliminate the fuel filter, fuel pump, etc. without just replacing a bunch of parts. I was suspicious of the filter and pump on this car, and a test that took 2 minutes to do ruled out those parts very quickly.

Speaking of, I got this to do the fuel pressure test -

Amazon Amazon
and
Amazon Amazon

Fit perfectly and did the trick.

Dale
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Old 04-28-19, 09:42 AM
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Bad connector (pin) connectivity is a common issue on F2000 ECU's. Unplugging and reconnecting usually corrects the issue. So what you found is not surprising.

Especially on racecars, which sit unused for relatively long periods between races, this sort of thing is quite common.
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Old 04-28-19, 02:37 PM
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Was the car driven a lot recently or had it been sitting for a while? The injectors may have been gummed up making one or more malfunction, sticking open, not fully opening. Once running the car for a while trying the different ECU's etc., maybe they starting working better. I would run a bottle each of good injector cleaner for the next two tanks of gas.
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Old 04-28-19, 04:35 PM
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I agree. The "stuck" injector was totally dead - put a meter on it you have an open circuit. I did click both secondary injectors open and closed with 12v during the process.

I'm leaning towards a bad connection to the ECU. Car is not driven often and I am going to recommend running some fuel injection cleaner. It really hasn't been driven much the past few months due to it running like crap with this issue.

Dale
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Old 05-01-19, 11:25 PM
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I've had the igniter partially fail similar to this, would run fine at idle/light throttle but as soon as it got any boost/load it would load up and pop/misfire until you backed off.
Never even thought of it since it never fully failed, i was replacing power fcs, redoing wiring, new coils, etc.
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Old 05-02-19, 09:48 AM
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Originally Posted by DaleClark View Post
- Tested fuel pressure - pressure stays around 38 psi, if I yank the throttle open and get it to stall it stays solid, no movement on the fuel pressure.

Thanks!
Dale
shouldn't fuel pressure change with manifold vacuum?
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