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BLUF: I'm getting weird either fuel or spark cut randomly throughout my entire RPM range on my 92 JDM FD
I've been away from here from a while due to a lot of work requirements that kept me away for a long time. Anyways, I just transferred earlier this year before Rona hit, and then was doing a lot of teleworking and staying at home like most people. I'm back to in-person work full-time now and took my FD out to drive in last week after it's been sitting for a while (3rd time since Apr). The first thing I noticed was when I was merging onto the freeway, I had "lost power" in accelerating, but wasn't sure what was going on. After a while when it kicked back in and once I got to traffic speed, and after a bit of cruising, all of a sudden, I would start slowing down and the tachometer would slowly start dying down, and then I would get "bucking" in the car after a little while. This would continue for a while as I would driving and I tried a couple of different things when it would occur. If I were to just play with the throttle, it would just make the bucking worse once I finally got power back. If I engaged the clutch it would sometimes making it better, and if I shifted, it would generally correct it. From playing around with stuff, for some weird reason, it seemed like in 4th gear it wouldn't occur as much. It was early morning and I didn't notice lights flickering during this, so don't think it's an alternator/battery issue but could be wrong, battery was dead when I hooked it up a few days prior b/c I forgot to disconnect it while it was sitting (Charged right back up with my recovery charger though). It was also really foggy too though, so didn't know if that played a part with moisture being a variable, but I had the same experience on the way home.
I went ahead and changed the leading plugs this weekend, even though I don't have a lot of miles on the current ones. While I changed them, I checked compression, and everything looked good with the front rotor: 108 / 109 / 110, back rotor: 113 / 116 / 108. Plugs looked good too, and I attached the leading plugs below. None of the parts stores had trailing plugs in stock, so didn't change those, but they were just a little less black than the leading (about 50/50). When test driving afterwards, it seemed like the hesitation was occurring less often, but still definitely there. I would do a few WOT pulls on the highway, sometime fine, sometimes not. For entrance ramps, it was usually not OK, and would cut on me. So, since the compression checked out good, I'm leaning on it being a spark or fuel issue.
Next thing I'll do is run through all of my grounds, and make sure that everything has a good clean contact. I have lived in humid environments since owning this car, so I know corrosion on those contacts is definitely something that could have built up on it. Last time I changed my fuel filter was a couple years ago, but it was also only ~5k KM ago. I also have yet to dig through the stock rats nest for any potential bad hoses within there.
None of these issues occur while at idle, and the car is pretty much stock except for a cat-back.
Just looking to see if anyone might have any guidance on where to maybe check next.
Welcome to "Limp Mode". JDM don't have a check engine light. Instead the ECU throws the car into Limp Mode if it detects a problem. Limp mode makes the car perform as you have described.
The intention is for you to take the car to the dealer.
You need a Japanese spec code reader. It will tell you what the ecu is thinking is wrong.
There are about 75 codes. Many will trigger limp modes.
You can also count blips to understand which codes are being triggered. You need to get a list of those codes. I do not know if they would be the same as the list in the USDM FSM.
If you have an original 1992 Rats Nest and the wiring is crispy, you likely are blowing codes all over the place.
If your engine bay has been simplified by removing a lot of the emissions and twin turbo solenoids, vacuum hoses and wiring, the stock ECU might not like it, and randomly throw you into Limp Mode.
You may want to do a smoke test of the air management vacuum line system. This will help identify any leaks in your system. The code reader might not pick up such leaks.
A shop with a smoke testing devise will hook it up to you system and pump smoke into it.
I had four leaks identified even after I had replaced all gaskets and hoses.
Cost was $50.
Have a plan before you start replacing vacuum hoses etc. Many of the solenoid nipples will break off due to age. New solenoids are $100-200 each.
I suppose you have had a fuel line pressure test. I had a bent o-ring near the pump which cut fuel intermittently .
The fuel pressure regulator needs vacuum to be on to remain off. So if that vacuum line is damaged the FPR may not be working. Likewise the fuel pulsation damper has a little rubber seal that could well be kaput.
Replace the O2 sensor ($50). Check that the Map sensor filter is pointing in the correct direction. Check if TPS has got out of range. Check with the engine cold and hot.
The TPS set/attachment screws tend to strip and are hard to reach. Try using a small ratchet with a Phillips head socket. The screws like to drop out of sight.
TPS are very expensive to replace and essentially you are hooped if you damage it.
Last edited by Redbul; Oct 27, 2020 at 11:42 PM.
Reason: added info., corrected info
You can also count blips to understand which codes are being triggered. You need to get a list of those codes. I do not know if they would be the same as the list in the USDM FSM.
[
TPS are very expensive to replace and essentially you are hooped if you damage it.
Apparently there is now a digital TPS available out of Japan. Cost $500~700.[/QUOTE]
there is a New Mazda part number also, its buried in that thread about the TPS substitute, it has been available for about a year or so. Mazda USA didn't get the memo, so the traditional vendors won't have it
Appreciate the input. I'm been slammed with work this week, so haven't had an opportunity to take a look. The gas is old (6mo), so that could definitely be a contributing factor. I always keep pre-mix in the fuel, and the car is stock except for the cat-back. I have it under a 1/4 tank now after the last time I was out, so thinking to either drain and fill with fresh gas, or top off what I have now, and then start looking around at some of the electrical and the ECU.
Appreciate the input. I'm been slammed with work this week, so haven't had an opportunity to take a look. The gas is old (6mo), so that could definitely be a contributing factor. I always keep pre-mix in the fuel, and the car is stock except for the cat-back. I have it under a 1/4 tank now after the last time I was out, so thinking to either drain and fill with fresh gas, or top off what I have now, and then start looking around at some of the electrical and the ECU.
check it for codes first, its easy, and it'll point you in a direction
So, got my LED's in last night so I could try to read the code, but not working. Tested the grounds in the diagnostic box for continuity and they are good, and the B+ is supplying 12V. I checked the voltage from the B+ -> FEN while the LED was in just to make sure too, and it stayed at 0. So looks like I need to track down whether the ECU is getting that signal or not. Looks like what Redbul was saying with the ECU going bad might be a case. Hopefully goes well this weekend.
ECU itself looks to be in good condition. All the components look really good, and no burn marks noticed anywhere, or electrical burn smells when opening it up. Also, all the solder joints look really good.
No bad capacitors from what I saw. Everything in there looked in great condition. I haven't plugged it back in, b/c I ran out of time last night due to other things. I'm gonna test the wires heading down to the ECU to make sure everything is good on those test wires before putting it back in.
Some codes will clear simply by turning the car on and off. But they will be triggered again once the ecu cycles through its checklist. I had an electrical fault with my OMP, that was intermitant. Sometimes it would show a code and other times it would not.
A fault with the OMP definitely triggers Limp Mode.
Limping to the Dealer
