Serious misfiring after a short drive [Archive] (2024)

MX-5 Miata Forum > NA/NB (1990-2005) Miata > NB (1999-2005) General discussion > Serious misfiring after a short drive

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I have a 99 with a Fast Forward Supercharger among several other updates. Recently after winter storage it started to miss periodically which progressively got worse. to the point it would barely go forward.Initially it was after a longish drive, but progressively got shorter. As mentioned it seems to be heat related as now I can drive it 10 minutes or so from cold and it will start to misfire predictably. It's gotten worse in other words. After discussing on the FFS supercharger forum here, I had the coil pack replaced as it seemed like a logical candidate and I have replaced one on this Miata before. That didn't solve the issue so I replaced the wires and spark plugs as pretty easy to try. Again no resolution. I finally bought a code checker and PO122 or low voltage from the throttle position sensor was displayed. I replaced the throttle position sensor and again after a 10 minute drive it started to miss like crazy. It does seem fine for the first 10 minutes however. Again I am getting the P0122. Not sure where to go next. Tom from FFS suggests replacing the 02 sensors.. He prefers Denso brand.l They are probably original with 119,000 miles. Any input would be appreciated. Thanks

Just thinking the whole situation over from the initial misfires until now. Whatever the failure is caused by has gotten worse as originally (after storage this spring) I would only experience a couple of misfires and ran without issue for many miles. Then at one point after driving around 10 miles from home or so it started to really miss and could barely make it back. After replacing the coil pack I went for a pretty extended drive without issue until again severe missing. Then after replacing the wires I could only drive a short distance before severe missing. The same after changing plugs and again after replacing the TPS. After cooling down you can drive it until the engine is just getting fully warm and it will start to miss severely. At least it seems that way to me. The PO122 code is specific to the TPS, but as Tom suggests it could be a wire breaking or failing, and I suppose that could still reflect as a TPS not working issue as while it is cold the TPS low voltage does not affect performance, but as it warms the computer changes the air fuel mix to an inappropriate setting as it isn't getting proper information from the TPS. Just guessing however. Not sure why it would progressively get worse over many days with a bad wire. PO122 isn't a code that comes on when some other problem occurs(from what I can tell from this site anyway). It is getting low voltage from the TPS. It is kind of hard to test the signal voltage when the TPS is connected as the wires are not accessible. Some techs use a pin to stick in the wire and then hook the volt meter with alligator clips to the pins. Not sure if that is possible or reasonable in my situation. As mentioned earlier I did confirm am getting 5 Volts to the TPS. Could replace the ECU as it looks pretty staight forward to remove and replace and I see one used for $100 from a reputable supplier.

The history is worth reiterating: Never really missed at all until after 5 month storage this year and then missed a few beats on first start up and then ran perfectly. for a tank of gas at least. Then after a short drive it started to run really rough, but managed to get it home. I thought it was the coil pack as that is super common on the 99 model. Put Techron in as well. The shop found several codes including TPS low voltage, miss on cylinder 2 and P1523 (air inlet solenoid malfunction).They also checked compression and found cylinder 2 with 150 pounds which is low compared to the other 3. It ran smoothly in the short 2 mile drive to the repair shop. After replacing the coil it seemed fine for a maybe 10 miles and then acted up again. When I checked for codes it would only should the TPS low voltage issue. I replaced plugs and wires and again it ran smoothly for a mile or two and then started to run very roughly with again the PO122 TPS code. Replaced the TPS with same results. Tested wiring for TPS and seemed fine when cold. Sent the ECU in for possible repair and they found issues and replaced leaky capacitors. After reinstalling the ECU it ran rough at start up and shows a miss fire on cylinder 2. Switched 1 and 2 sparkplugs and it ran smooth for a few seconds and then rough again. So it seems to have progressed from a few misses after storage to no cylinder 2 at startup.

Thanks for the tip! I did use the full tank I had in during storage and fill it again so it is probably clear of any water issues.

I purchased an inline spark testing device. I put it in line with #2 spark plug and started the engine. It ran all all cylinders for a few seconds and started to miss which is very similar to yesterday after installing the repaired ECU. The light on the device was bright and steady when it wasn't missing and also when it was missing. I tried inline with #1 for a comparison and it looked basically identical to #2. So whatever the problem is it is probably not associated with the ignition side of things. I am pretty sure the exhaust smells like unspent fuel. It certainly isn't normal smelling. There is Techron in the tank. Not sure where to go next. Perhaps sell it as is and look at a newer D series. Might be able to get the recent repair expenses back out.

I bought and added a can of heet to the fuel and tried it after a few hours. It still missed severely. Historically it does seem coincidental that the missing really started on the same day of the last tank of 93 octane (BP) I filled it with. It's not drivable with the heavy missing at this point. If it is or was water in the tank would I have to run the engine it to clear it out or will the heet clean things up just sitting? I am next considering to replace at least the number 2 fuel injector as I am consistently getting a PO 302 code.

I just tried it again about 24 hours after adding the heet. This time it ran smoothly and definitely on all cylinders so I drove it around the block. After a few minutes ( I did not want to drift too far away) the familiar miss started again, perhaps not quite as bad as yesterday, but seemed to worsen as the engine warmed up. It backfired slightly at one point in the short drive. It definitely was one cylinder not firing for part of the time anyway. I got back to my house and it hadn't thrown any codes and the CEL light was off although it was missing pretty steadily at that point. AS mentioned when I first started it a few days ago after re-installing the ECU it started missing very obviously right away and I didn't drive it anywhere. This time it ran smoothly for a few minutes and then seemed to slowly start to get worse.

In post #12 you tested #1 & #2 for spark. You needed to test #3 & #4 for continuous spark too. The way your car is acting, you shouldn't assume anything or leave anything to chance.

I am guessing that the computer changes the flow at the injectors as it warms up and that is when things go sour with the #2 cylinder/injector. Even though it has missed heavily instantly on start up after being off for a day or 2 as well. I am trying to remove the injectors, but it looks like releasing the fuel connection to the rail is in a very difficult spot to get at with the supercharger in place. Anyone else changed injectors with an FFS installed? There are a couple of pictures attached to this thead on the FFS section of Miata.net https://forum.miata.net/vb/showthread.php?p=10804121#post10804121

Took a break from working on my 99,but looking at it today. I am trying to remove the fuel injector rail and the connection to the fuel line is not cooperating. I have two or three of the tools for releasing the fuel line and although I seem to have it in place and push on it the line stays intact. Anybody have any experience disconnecting the line? The Supercharger makes it a bit more difficult to get at, but I can put the tool in place (and it fits) and push on it without too much difficulty. Thanks

Finally got around to addressing the miss again this spring. I found a great Miata specialist in Princeton, Minnesota which is about 60 miles north of me. Miata-Source picked up the car with a trailer and took it to their site and after checking compression on all cylinders felt that although there was obvious wear the engine should go for awhile longer. They found one of the harnesses for the injectors had an intermittent short and replaced it with a used one they had. That fixed the problem! They went over the vacuum and gas lines and replaced several of them. They just dropped it off yesterday and after getting a fresh tank of 93 octane, I took an extended drive and the car runs great! Looking forward to a few more summers of driving my 99! Check out miata-source.com if you have time. Zach and crew are very capable. Thanks to everyone for the help during this stretched out repair process.

So was it age or mice causing the short on the injector harness?

The connector at one end had been rubbed against over time. Zach felt after installing the replacement that it would not happen again. See attached picture.

Not something that is typical or expecting to see like a malfunctioning CPS.