Throttle issue at certain temperature

[kaur]

I have this issue when the car seemingly acts up at a certain temperature before warming up. It works well when I start it cold, but depending on ambient temperature, in a couple of minutes throttle acts weird until the engine warms up further. When this problem occurs, throttle response is strange. At first, when I depress the pedal, the car accelerates; when I press a bit more, nothing happens; when I press even more, I get a surge of power and it accelerates again and at higher revs everything is OK. It happens somewhere between 1000 and 2000 rpm, so when I'm cruising, I can lose a bit of speed and when I adjust, I also get like this jumpy acceleration from nothing to a lot.

The car is also shaking a bit when idling with the engine warm.

The ECU gives me codes 21 and 41.

Just recently I replaced a bunch of vacuum lines but didn't really manage to do all of them.

I tried to check the CHTS but I'm having issues figuring out which connector it is and the leads of my multimeter are too thick to check it on the ECU connector. I got a 5.1 kOhm reading on some terminals I thought could be the CHTS connector but I'm not sure; the engine was warm then. I am going to try again to find a connection to the CHTS I can measure the resistance at.

It's a European 1987 turbo version; it has some small differences from the US ones.

Reading about similar issues have made me suspect the CHTS but could it also be something else? The error codes don't seem to indicate any problems with it.

[kaur]

The CHTS checked out fine both warm and cold. Following the lead from code 21, I checked the FTS. It's resistance seems okay but I don't get the diagnostics part about it in the FSM. It says I should check the voltage between ECU terminal 15 and ground while the engine warms up from cold. Is the voltage supposed to go from 5 to 0V or just be in the range? I get 3.67V with a cold and 3.5V with a warmed up engine. How do I go about checking the FTS and code 21?

[kaur]

So I hooked up a USDM 86 turbo ECU and the few times I test drove it starting from cold the issue appeared to be gone. The car will be sitting for some time now for other reasons so I can't fully confirm it but I think it was an ECU problem. Either a mixture problem in a certain temperature range or just a dying ECU.

[amreboot]

It's not 100% shocking that a faulty ECU can cause this issue.
However, when in doubt, replace the CHTS and subharness for it.
These sensors are very known to cause issues very similar (if not exactly) to what you have described above.

[kaur]

I was close to ordering a new CHTS, but because it's resistance was quite well in line with what it's supposed to be I didn't want to replace a working part. I was even considering replacing the FPR because of the FTS error code, but it also appears to be working fine. However, even the FSM gives a rather wide range for the resistances so it may be close to the lower or upper limit there and the difference from the average can be large enough for the ECU to misinterpret it. Maybe I'll have to replace the CHTS anyway, time will tell.

For now it seems a USDM 86T ECU works on an EDM 87T. It did seem to crank just a bit longer before starting, but it may have been random. I'm not sure if I'll pass the emissions with it though..

[kaur]

Just for future reference, I ran the car with the 86 USDM turbo ECU the past season and this problem did not appear once. It's probably a difference in some warm up settings on the Euro model ECU or something like that.