Help needed with a EFI vehicle

[flyology]

My runaround car (a 1990 nissan pintara U12 with CA20 and 5 spd) has been playing up a bit lately, seems to be a cold start issue? when it is cold it starts no problems, turn the key and away it goes. however if it is hot, or hot outside, it will only start if you use a bit of throttle, which to me on an EFI car should not be done. I did have some issues with the temp gauge on it (temp gauge will not read correctly unless a factory sender is used) eventually got one and its now ok.

I also changed the temp switch for the thermo fans for an aftermarket switch, possibly this is the issue, but i cant find the original one or get another factory one at this stage. I cleaned the AFM it runs better but still has the start issue.

Any ideas?

[Loanrangie]

There should be a temp switch that tells the ecu to enrichen the fuel mixture for cold starts , if this fails it usally gives problems on cold starts as well.

[DamTriton]

Idle air control motor may be shot, and stuck in the "cold" state effectively setting the mixture to "rich". This would explain why you have to open the trhottle valve a bit to get it to lean out enough to have a go at starting in hot weather.

My guess is it is also consuming more fuel than it should too...

[flyology]

Idle air control motor may be shot, and stuck in the "cold" state effectively setting the mixture to "rich". This would explain why you have to open the trhottle valve a bit to get it to lean out enough to have a go at starting in hot weather.

My guess is it is also consuming more fuel than it should too...

this sounds exactly right!

Any suggestions on where/what to look for the air control motor?

[DamTriton]

Idle air control motor may be shot, and stuck in the "cold" state effectively setting the mixture to "rich". This would explain why you have to open the trhottle valve a bit to get it to lean out enough to have a go at starting in hot weather.

My guess is it is also consuming more fuel than it should too...

this sounds exactly right!

Any suggestions on where/what to look for the air control motor?

Should be near the throttle valve with a couple of hoses (or cast in air channels) either side of the throttle valve, and an electrical plug connected to it. Iys purpose is to bleed air across the closed throttle valve to allow the engine to idle.

It will be seperate to the throttle position sensor (which will not have hoses to/from it). You may also find an inlet air temp and mass airflow sensor in the general area too, but with the big inlet rubber hoses off you easily identify these.

First things first though, try giving the contacts to the motor and all electricals across the inlet manifold a good clean.

This is generic information only as I'm not to sure about exactly how your system is set up. I'm a nurse, not a mechanic, but I have experience with EFI's

Have you been able to retieve any error codes from the ECU? This would help.

CA20 ECU Codes

code: description:
11 engine speed sensor (*)
12 system pass
13 oxygen sensor circuit (open)
14 coolant temperature sensor circuit (voltage low)
15 coolant temperature sensor circuit (voltage high)
21 throttle position sensor circuit (voltage high)
22 throttle position sensor circuit (voltage low)
23 start signal circuit (*)
31 park/neutral switch circuit - auto only (*)
32 air mass sensor circuit (voltage low)
33 air mass sensor circuit (voltage high)
34 hotwire self cleaner (*)
43 vehicle speed sensor circuit
44 oxygen sensor circuit (lean)
45 oxygen sensor circuit (rich)
55 ECU

Note: (*) the ECM lamp will not light for these faults.

Note: these codes are applicable to the Australian U12.

A Gregory's manual should tell you how to retrieve the codes (usually jumpering a couple of pins in a socket, or holding a particular dash switch in while you turn the ignition on). It will cause the check engine light to flash on/off in a sequence that can be read.

You may need to disconnect the battery to clear all the codes that have been stored, and to allow the ECU to "relearn". It will be normal for performance not to be good for 5-10 km after doing this while the ECU sorts itself out.

[Ruffy]

Idle air control motor may be shot, and stuck in the "cold" state effectively setting the mixture to "rich". This would explain why you have to open the trhottle valve a bit to get it to lean out enough to have a go at starting in hot weather.

My guess is it is also consuming more fuel than it should too...

The idle air control motor simply allows air to bypass the throttle body. If it was stuck in the "Cold" position this would mean that the valve would be open, allowing more air to enter the engine. Nothing to do with "Richening" the mixtures. The increase in air flow allows for a higher idle. The ecu detects the Engine coolant temperature and increases injector pulse with to allow extra fuel on cold starts.
I'm going to break the "diagnosis rules" here and presume that the ignition system is ok.
Opening the throttle will do two things, allow more air, so if you have a rich start mixture you will allow more air in compensating for the rich mixture. The start mixture is a preset value and the only inputs that can alter it are Engine coolant temperature and intake air flow (measured by Air flow meter). The second is that the throttle posistion sensor will detect an open throttle percentage and adjust injector pulse width accordingly. So if you have a lean start mixture then you will effectively be richening the mixture to an acceptable level.
I know it sounds strange that you are richening and leaning at the same time but the more open throttle is measured exponentially to the idle mixture so although your are giving more fuel by opening the throttle the air fuel ratio is lowered.
Does it blow any smoke on start up hot? If it is a rich start scenario you should get a puff of black smoke.

[geoffro46]

check for fuel in the vacuum line of the fuel pressure reg also.may be leaking fuel into manifold and causing it to be flooded.also makes you lose fuel pressure so hard to start.

[flyology]

Ok, thats for all the advice, this afternoon I had 5 minutes spare to have a look and checked some of the connections around the throttle body. the plug into the TPS appeared to not be in far enough, but on taking it out and putting it back in a few more times it didnt really go in any further. Checked the wires on the idle motor and gave them a bit of a wiggle.

On driving to work tonight the car started first go, and ran a hell of a lot smoother. (driving with the aircon on was like driving with it off previously) On arriving at work I turned it off and started it back up a few times, no need for any throttle manipulation. Obviously there was a bad connection somewhere, so I will check again tomorrow and see if any of the plugs need replacing/fixing.

thanks again for all your assistance!!