21 Turbo R21 turbo starting and idle problems

[LauriS]

Hello from Finland! My 1988 21 turbo is having starting and idle problems and I haven't found out the reason for these problems. I have already asked these same questions in Facebook 21turbo owners club page and now I try here too.

Problem is:
Starting is slow, it takes few seconds (maybe 3-7 seconds) before the engine starts with a small cloud of black smoke. I think it gets too much fuel, it usually helps if I push the throttle fully open when starting. Not much difference if the engine is hot or cold, when hot it sometimes might start a bit better.

Idle problem is worse when engine is hot but it is there also with cold engine. Engine runs normally about 20 seconds and then revs go slow (about 500 rpm maybe) and start to to go up and down for a moment before it calms down. That happens every 20 +/- 5 seconds. Otherwise the car works perfectly, accelerates well and no problems when driving.

Things I have done allready:
-new water temp sensor (Bosch sensor that should be right kind)
->water temp sensor seems to give about 15-20c higher readings than the water hoses are in reality, is this difference significant?
-new sparkplugs (NGK TR7IX gapped to 0,55mm)
-new TDC sensor
-tried two used idle control valves (from R25turbo, same part)
-tried another ignition coil
-tried another MAP sensor
-air temp sensor gives reasonable readings when checked with xr25
-vacuum and air hoses/pipes checked carefully
-unplugging knocking sensor doesn´t affect anything
-fuel pressure checked, 2,2bars
-I went to check CO which was about 2,9. It was impossible to adjust right because the idle problem. C0 should be 2 I think?

Any ideas what can cause these problems and what should I do next??
I do have XR25 with me occasionally and I'm trying to learn to use it. Is there something that I especially should look with that? At least all the sensor readings look about right.. If I remember correctly there was some memorized fault(? blinking box) which I have to check what it was.

Actually I don't even know if there's two different faults at same time or is this all just one fault.

 

[Brigsy]

If the coolant temp sensor is reading wrong that needs sorting as its essential for the ecu to know correct temperature for enrichmenfs.

@DaveL485 will be able to advise which one is correct.

Also these have throttle position sensor from memory, check that is set up correctly and working

 

[LauriS]

I believe DaveL485 is the man who advised me at Facebook with the coolant temp sensor I have fitted "Bosch Temperature Sensor NTC M12" (0280.130.026) I also have tried another aftermarket sensor and that made no difference.

The throttle position sensor seems to be ok, at least XR25 gives reasonable readings. I have tried to adjust the throttle bypass screw on the throttle body but that really didn't help.

This is really a mystery to me so any ideas what to do or check next are welcome!

 

[DaveL485]

@DaveL485 will be able to advise which one is correct.

The OE turbo coolant sensor is a different resistance range to the non-turbo models, and they are NLA now. Every aftermarket part thats listed is the non-turbo type, which is in the 3000-ohm range to the turbo 300-ohm range (or maybe the other way around, still, its miles away). The Bosch sensor above is the closest replacement I can find with the right thread, plug and sensor ohm range. I did say it wasnt perfect when I suggested it but its better than a non-turbo one or a broken one.

The idle instability is common, you can usually improve it a bit by adjusting the throttle air bypass valve and cleaning the idle valve out with brake cleaner then washing it out with WD40.

 

[LauriS]

Yes the Bosch sensor may not be perfect but it seems to be very close. If my measurements makes sense and it is true that real water temperature is about 20 celsius less than ECU thinks makes me wonder can this small difference really cause the problem?

Before I was aware of this non-turbo/turbo water temp sensor difficulty I bought "stark skcts-0850052" sensor that was supposed to be right for the turbo. I think I will measure and see what kind readings that gives compared to Bosch sensor and real water temperature. Even though that most likely isn't the right kind..

I have already tried two idle control valves but I have one more on my R25 V6 turbo which I'm driving now, so I think I will try that too because it surely works fine.

I also tried to adjust the throttle air bypass valve as you advised in Facebook but maybe I didn't open it enough. How much can I adjust/open it? Before I touched it XR25 value for throttle position sensor was 6. I did open it so that the XR25 value was 10 and that didn't really help so I turned it back to 6.

Water temp sensor and idle valve seem to be the most obvious reasons. And since the OE coolant sensors are not available its hard to be sure it's not that.
Could it anyhow be ECU problem or fuel injectors? I think unlikely, but on the other hand it feels that everything is possible in this case

Thank you for your replies. All ideas and information is much appreciated.

 

[DaveL485]

throttle air bypass valve

This is not the throttle butterfly backstop - thats what you were adjusting if it affects the TPS. The air bypass is on the other side of the throttle body, and there is a plug in the hole it sits in. Ill have to get a picture this weekend.

 

[LauriS]

Oh, I did misunderstood that. Thank you for correcting this, I will try that next time I'm with the car.

 

[DaveL485]

This is the air bypass screw

 

[LauriS]

This problem is finally solved. Fuel pressure regulator was leaking fuel to the inlet manifold through vacuum pipe and that messed up the air fuel ratio.

 

[DaveL485]

This problem is finally solved. Fuel pressure regulator was leaking fuel to the inlet manifold through vacuum pipe and that messed up the air fuel ratio.

Thats the first time I have head of a 21 fuel reg failing in all my years messing with them. How unusual!