1990 300zx N/A High Engine Idle (MT)

[Jevitto]

Hello I have a 1990 300zx N/A (5 spd MT) and I just bought it recently with a salvage title, the car has 165XXX miles and it's currently having issues.

The problem is the engine idles around 1500 rpm and sometimes 2000 rpm.

There is no check engine light on and just to double check i ran the ecu diagnostics and it reported code 55 for "everything is ok" (5 slow blinks, 5 fast blinks)
Since I have a code 55 i'm assuming i can rule out the following list of culprits
Codes under Mode II
11: crank angle sensor circuit
12*: air flow meter circuit
13*: engine coolant temp. sensor circuit
14*: vehicle speed sensor circuit
21: ignition signal circuit
26*: boost pressure sensor
31*: ECU
32*: EGR function
33*: exhaust gas sensor circuit (left)
34: detonation sensor circuit
35*: exhaust gas temp. sensor circuit
42: fuel temp. sensor circuit
43*: throttle position sensor circuit
45*: injector leak
51*: injector circuit
53*: exhaust gas sensor circuit (right)
54: signal circuit from A/T control to ECU
55: NO malfunctions in the above

I have cleaned out the throttle body and checked for vacuum leaks using throttle body cleaner around all 14 vacuum hoses.

I checked the spark plugs by removing them one at a time while the engine is running and they are all in working condition (NGK Spark plugs)

O2 sensors work fine

I've adjusted the IACV screw and it's turned clockwise as far as it will go.

The TPS is working fine .45V

The throttle return spring is a little worn out and replacing it would only reduce my rpm by about 200-300 which still wouldnt put me near the 750 rpm mark.

The throttle cable is where it should be and working well

i have yet to check the fuel pressure but i doubt thats the issue

I've checked alot of things and still have more to check but i'm hoping maybe one of you guys can help me figure this one out


Another problem i'm having is trying to go into 1st gear when the car is stationary but all other gears work fine, i thought it was the slave cylinder but i checked it and it's not so hard to push in the rod with my hand so i'm assuming i can't go into 1st gear EASILY because of the high rpm 1500-2000 rpm. When i'm moving around 5 mph or higher, the car goes into 1st without any grinding.

Anyways, hope to hear from you guys soon and thanks in advance

P.S. I've been researching on the internet for about a week now and couldnt find one post that helped me with my issue so i just made my own post, sorry but thank you

[t.mcginley.jr]

The IAA unit itself could be bad. There's a how-to on how to test it I believe, sometimes they stop working.

Could also be an issue with your MAF.

[Jevitto]

The IAA unit itself could be bad. There's a how-to on how to test it I believe, sometimes they stop working.

Could also be an issue with your MAF.

I've cleaned the MAF and the electrical connectors work fine, usually with a faulty MAF you get low rpm or even an rpm that fluctuates but either way thats not it.

I've tested the IAA and IACV by turning the car off, removing the negative terminal and putting the car into the on position and not the start position, i then touched the negative terminal with the car in the On position and listened carefully, the IAA/IACV sound like they are both working.

This doesnt mean it works perfectly and I have yet to inspect the inside of the unit itself, so I will be checking that today and cleaning it out while it's off the car

I will let you know if this fixes the problem and thanks for your input, much appreciated

[t.mcginley.jr]

First, are you sure the throttle cable isn't too tight (two adjustment bolts).

Second, vacuum leaks can also cause high idle, as can a bad coolant temp sensor. Vacuum leaks require a boost leak tester to find, the temp sensor can be tested just like the TPS, you'll have to check the FSM for the voltage values though.

[Jevitto]

First, are you sure the throttle cable isn't too tight (two adjustment bolts).

Second, vacuum leaks can also cause high idle, as can a bad coolant temp sensor. Vacuum leaks require a boost leak tester to find, the temp sensor can be tested just like the TPS, you'll have to check the FSM for the voltage values though.

Throttle cable has enough slack to function properly so thats out of the question

I will get my hands on a boost leak tester to double check for vacuum leaks

I will test the temp sensor after I check the IACV/IAA but one question

Where are the release clips on the IACV/IAA Electrical connectors because i'm having a hard time removing them

again thanks for your input

[Jevitto]

UPDATE

I just pulled off the IACV/AAV and i removed one of the hoses

Upon removal i discovered one of the metal tubes was bent

It was not bent all the way shut but its definitely decreasing the amount of air flowing through it

One of the previous owners must have bent it somehow

Anyways stupid question but.... Do you think this is what is causing my High Idle?

[t.mcginley.jr]

Anyways stupid question but.... Do you think this is what is causing my High Idle?

It's possible that it was messing up airflow to the unit and tricking the ECU into thinking it needed to compensate. Can you bend it back or is it all jacked up?

[Jevitto]

Anyways stupid question but.... Do you think this is what is causing my High Idle?

It's possible that it was messing up airflow to the unit and tricking the ECU into thinking it needed to compensate. Can you bend it back or is it all jacked up?

It's made out of brass so i was able to bend it back to a more decent position but will never be good as new...

I'm putting it back together right now so we will see how she idles

i'll let you know how it goes

[Jevitto]

I've hooked everything back up after fixing the IACV tube but i still have an idle of 1200 rpm

anything else i can try?

[Jevitto]

Well... I have checked everything that my tools have allowed me to check and I think it's time to take my car down to a mechanic with lots of experience in imports and all the tools you will ever need

Thanks for trying to help me figure out my high idle issue

I will let you know what the problem was when I get my car back from the mechanic if I can find one i'm comfortable with

[t.mcginley.jr]

Sounds good, sorry we couldn't get the issue sorted without resorting to going to a mechanic