Ecu issue?

I’ll start by mentioning that i had the car apart for an injector swap with overspeed seals, added a perm pcv plate, new oem pcv valve, and a cs sealed catch can with a check valve. Other than that, nothing has changed. I did not remove the intake or anything that may cause an ecu failure due to rubbing on any wiring.

The issue I’m having a few weeks after the install is the car will randomly start revving on its own, coolant tempt reads in the negative for a split second (ambient temps have been in the 70’s), and traction control light will come on all at the same time while idling. Other than that, no drive-ability issues. I’m sort of at a loss for what could possibly be the issue. I can see where the revving and a vacuum leek could be related. But negative coolant temps at the same time seams odd.

Also to note, I changed the coolant temp sensor a few months back with an oem one as well.

Anyone have ideas?

 

If you had the ECU out then look at the pins on the connectors.

 

I agree with Raider. If it was a vacuum leak then it would be more consistent. Check your grounds and also check the harness for being bound or pinched. If there is too much tension stretching a wire it can cause random electrical issues to show up. Easy way to test your ECT sensor circuit is to take a jumper wire at the sensor connector and bridge the pins. Monitor the scan tool data and see what it shows. It should default to some ridiculous value either + or - temperature.

 

Ah I didn’t consider the harness. I have noticed it is pretty crusty in some spots. I may end up grabbing a new anyway. I did not remove the ecu during the install but will still check.

 

Many weird electrical issues have been solved by unplugging the ECU and plugging it back in

 

Small update:
Last week I took a look around for any pinched or stretched wiring. I checked the ecu pins and wiped out all of the surrounding area. Made sure the intake clamp wasn’t rubbing on anything. Everything looked good. It seemed to be fine this past week. The only thing I noticed is the temps wouldn’t really rise above the 174 range, but didn’t think much of it since its getting colder out.

Then today I had only made it about a minute up the road and while stopped at a light the coolant temps shot down to the negatives again. Simultaneously the car started revving on its own to about 3k and stayed there so I shut it off. Then it wouldn’t start. Checked for codes and got these:
P0126
IC U2064
ABS U2023

When I got it back to my apartment I cleaned the ect connections and all other connectors on that section of the harness, including the ground near the hpfp. I also cleaned and wrapped areas of the harness that didn’t have any insulation. The wiring insulation is pretty dry and flakey but the connectors and wiring didn’t seem to be in bad shape. I didn’t have too much time to sit and wait for it to act up but for the few minutes it sat warming up, the coolant temps were still fluctuating. Not as drastic as before but it was still happening.

I don’t have a multimeter. Looks like I’ll be buying one and going through everything again. I usually try to stay away from messing with anything electrical so this has been the last thing I was hoping to be the problem.

 

Have you replaced the thermostat with an OEM one? I had an issue with my car throwing a P0128 code and the car taking a long time to warm up. I replaced the thermostat with an aftermarket one and had overheating issues (regular 3 Stat is a little longer than a Speed stat and won't full open) but once I replaced it with a OE one it went back to normal.

174 is low for operating temps. Another option is to buy a new ECT sensor connector and just run wires from that sensor straight to the ECU without touching the factory wiring. If the problem corrects then you know it's wiring.

 

Yes I forgot to mention I replaced the the thermostat and ect sensor at the same time. Both were oem from what I can remember. However I did go back and try to find my order from rock auto and can’t seem to find it. I had suspected this might be from not buying an oem sensor.

I’m going to order an oem sensor just to verify that’s not the issue. In the meantime I think I’ll try to bleed the coolant system of any air. Maybe there is an air pocket that’s throwing off the sensor readings.

That’s a good idea to grab a new sensor connector. Didn’t think of that. The previous owner seemed to have messed with some of the wiring at some point too. When I got it (6 years ago now) I noticed a few roughed up/slightly broken connectors and tape in random spots. I’ll have to see if there are any local nator guys willing to give me a hand if it comes down to replacing wiring. Thanks for the input!

 

Well it turns out the temp sensor I swapped a few months ago was in fact not oem. All of my issues and codes (P0126 & P0118) have gone away after installing a new oem sensor. I normally make sure I’m buying oem stuff for parts like this, but this ended up being one of many parts installed that day. There’s an obvious difference, but It must have been overlooked. I learned my lesson! Now time to verify the thermostat I installed at the same time isn’t another brand as well.

While troubleshooting the issue, I noticed the connector was pretty rough looking. I hit up @Eric@MazdaspeedWireTuck to see if he had a replacement and even though he’s a busy dude, he was cool enough to put together a new temp sensor connector and wiring. He’s the man! Regardless of the issue seeming to be solved, I’ll still be switching out that section of the harness with Eric’s. Once I buy another daily next year, I’m going to play around with re-doing a lot of the harness in my spare time since it’s pretty crusty at this point. I’m not great with electrical but I feel like that will be the perfect time to learn.
Thanks for the input guys!