Coolant temp problems

[Ryan S]

Hello, I’m new to Starquest’s so maybe someone can point me in the right direction of my problems.
 

My coolant temp gauge seems to work but very little. It usually goes just above the line above “cold.” Then when I start driving it goes to just about cold. If I sit still and rev it (instead of idle) for a little while it’ll start to get to the middle where I’m guessing it’s supposed to sit. I’ve replaced temp sensor for gauge and kinda looked over the wiring. Everything seems good. Is there a ground that could cause my problem for the gauge itself or should I go through the whole harness?

 

Another question I have is when I am driving and stop at a sign or light it’ll high idle around 1,200 rpm like it’s ice cold (which the gauge says it is but I know the gauge uses a different sensor) but drops back down to ~850 rpm after sitting for around 20 seconds like it warms up a bit. Could this also be a wiring problem? I’ve also replaced the temp sensor for the ECU. 
 

I have checked the head with a temp gun and it says about 185-190 so I know the thermostat is working.

 

Any information is appreciated, thank you!

[psu_Crash]

I wouldn't put a lot of faith in the stock temp gauge. They have been known to be inconsistant over the years. Grounds are likely a good place to start though. If you haven't already found them starquestgarage.com has all the wiring diagrams and the manual shows all ground points. Cluster and engine grounds would be my first go to, then start to clean body grounds. Any weird misfire or hesitations?

When it high idles while warm, are you running the AC? That can kick things up a bit. 

[TexasQuest]

For the idle issues you may need to perform a reset.

 

[Ryan S]

15 hours ago, psu_Crash said:

I wouldn't put a lot of faith in the stock temp gauge. They have been known to be inconsistant over the years. Grounds are likely a good place to start though. If you haven't already found them starquestgarage.com has all the wiring diagrams and the manual shows all ground points. Cluster and engine grounds would be my first go to, then start to clean body grounds. Any weird misfire or hesitations?

When it high idles while warm, are you running the AC? That can kick things up a bit. 

Ok thank you for the suggestion. I’ve deleted the AC as I really never use it anyway. As for misfires, yes… I have a problem with when it warms up it starts to misfire kinda, barely noticeable but still there, then when I try to hit boost it totally falls on its face around 3500 rpm’s. I have clips and injectors coming to hopefully fix that problem.

 

thank you!

[Ryan S]

4 hours ago, TexasQuest said:

For the idle issues you may need to perform a reset.

 

Do you have the link for an 86?

Thanks!

[admin_JAinsworth]

When it falls on its face around 3500 willl it pick back up? It could be the vacuum advance. Do you know how to test it?

Jimmy

[Ryan S]

5 hours ago, admin_JAinsworth said:

When it falls on its face around 3500 willl it pick back up? It could be the vacuum advance. Do you know how to test it?

Jimmy

No it acts as it hits a barrier. It won’t do anything, doesn’t build boost just stutters till you let off.

I’ve put a vacuum pump on it and it holds vacuum just fine.

thanks for the response!

[admin_JAinsworth]

 

[psu_Crash]

Will it free rev in neutral past 3500? 

[Ryan S]

On 5/9/2025 at 5:43 AM, psu_Crash said:

Will it free rev in neutral past 3500? 

Yeah, it’ll rev pretty good not under load. It’ll rev to around 5500 and does the stuttering a bit. But as soon as you put it back in gear won’t do anything past 3500. It seems to be the worst in 2-5 when the engine has the most strain on it.

 

thanks!

[Ryan S]

On 5/9/2025 at 2:49 AM, admin_JAinsworth said:

 

Awesome, thank you!

[Ryan S]

New problem… got the injectors and clips, took off the Tb, cleaned everything and put it back together along with all new vacuum lines.

My problem now is it won’t idle at all, and misses unless I keep taping/pumping the gas pedal like an old carb. But it revs perfectly and sounds good. I’m stumpt…

Another thing is when I took apart the Tb and took off the Idle air module, the screws looked like they were overheated to hell. They were all discolored like bluish, purplish. 

I’m still having issues with my coolant temp gauge as well as the actual temp sensor for the ECU I think. Because it smells like it’s rich which could also be my problem with idling. Anyone have any suggestions? Is there maybe a common ware spot for wires for the coolant temp sensors? 

Also if the thermo valve controls vacuum  for cold/hot and with the coolant temp sensor for the ECU (maybe not working) could this be a problem for my idle issues?

Thanks!

 

[FRO]

Sounds like you have multiple problems interacting at the same time. 

This is what I recommend you check.

Do an Ohm reading on the gauge temp sensor as well as the ECU sensor. Specs are in the manual. If the Ohm sweeps check out then it is likely that the engine temp is really low and the ECU is stuck in cold start mode. Is it possible that there is no thermostat installed? or is it stuck open? easy check: when the car is dead cold, pull the radiator cap and start the car. Once started, rev the engine and observe if the coolant is quickly moving in the radiator. If so, at a cold start the thermostat should be closed, and NOT allowing coolant to circulate quickly. 

I would try to solve temp issue first, then work on idle.  

Hope this helps,

FRO

[Ryan S]

14 hours ago, FRO said:

Sounds like you have multiple problems interacting at the same time. 

This is what I recommend you check.

Do an Ohm reading on the gauge temp sensor as well as the ECU sensor. Specs are in the manual. If the Ohm sweeps check out then it is likely that the engine temp is really low and the ECU is stuck in cold start mode. Is it possible that there is no thermostat installed? or is it stuck open? easy check: when the car is dead cold, pull the radiator cap and start the car. Once started, rev the engine and observe if the coolant is quickly moving in the radiator. If so, at a cold start the thermostat should be closed, and NOT allowing coolant to circulate quickly. 

I would try to solve temp issue first, then work on idle.  

Hope this helps,

FRO

Thank you FRO,

I think it has a thermostat as I’ve put a temp gun on the head and it reads 185-190 so I’d assume there’s a thermostat. I’ll have to do your test. 

If I remember correctly when you first start it, at dead cold, it does high idle around 1100rpm and slowly comes back down to ~850rpm. So honestly I think cold start is fine but that still doesn’t rule out why it high idles when you come to a stop after cruising, but then slowly goes back down to a normal idle when you sit there for a a minute.

Plus now I still have to figure out why it doesn’t idle at all unless I’m on the gas pedal.

Thank you guys for the responses!

[kev]

There are many things it could be, but being that things changed after the new injector clips and TB cleaning, I'd place focus here.....but, take into consideration what was said on a previous post.   You could be stuck in closed loop.   

If you unplug the CTS (the one that goes to the ECU and is vertical with the prong facing up) and put a 200 ohn resistor between the two terminals of the plug, it will trick the ECU into thinking the car is fully warm.   Obviously, you will have to start the car and let it come to temperature to do further testing...but this will rule out CTS issues as you further diagnose.   

If still having idling issues, now start going through what you have done.  Did you make a mistake on soldering in the injector wires...inadvertently switched a wire, cold solder joint, etc.?   Did you replace all o-rings using a TB seal kit and NOT use any type of sealant that could have gotten into an injector port?    Did you attempt to do a TPS/iSC reset...probably not being that you can't get it to idle, but have you moved the TPS to see if it makes a difference?     Did you inadvertently touch the idle screws on the TB?     On that note, there are two idle screws...one for the ISC, which is the primary idle adjustment...the second on the throttle plate itself, which is the secondary that should be set a turn or so less than the ISC to 'catch' it if the ISC allows the idle to drop and not stall the car.    Note: there is a popular youtube video out there that shows the incorrect way of setting the idle by turning the fixed idle screw on the throttle plate....although it is incorrect, you can try that first to see if you can get it to idle but know that you will have to go later back it off and go through the TPS/ISC reset procedure in very good detail to get the electronics and the idle properly set right.  

To me, the new idle issue seems to be either a bad connection on the new injector clips or a blocked injector.  Also note that the black injector is known to gum up if you let it sit awhile.  The green injector only opens when you push the accelerator, as soon as you let off or hold it still, it will stop spraying.    So, if your black injector isn't opening, it will stall out in these conditions.    I greatly suspect this is your issue...confirm it and if it is, find the cause..it's either gummed up, not getting the signal to open, or blocked. 

 

As to the temp sensor for the gauge, if it is an aftermarket sensor (assuming so since you said you changed it), they are known to have different calibrations.   I've tried a couple different aftermarket ones from rockauto and ended up with ones that show the car almost overheating and another that showed it barely heating at all...yet the car was running at the proper temperature.   I always ended up back with an old OEM one.    I'd also be very cautious on an aftermarket ECU CTS. 

 

[kev]

Also, I don't like that version of the TPS/ISC reset that was posted above.  I follow the one in this thread (the original posted by KO but is mike c's wording...the 9 steps).     I know, there are so many but most have forms of simplification which can run you into problems.   Follow mike c's reset procedure to the T.

 

 

[techboy]

Another thing to add onto the pile here ... are you still running an OEM FPR?  If so, it's probably pretty old at this point.  I ran into an issue just 2 years ago where my car was stumbling at 3500ish RPM's like yours.  I tried all kinds of things before I finally bought an aftermarket FPR and installed it. (I was resistant b/c I was trying to keep the car as OEM as possible)   Car ran great.  Here I was chasing a problem all over the place when it was a weak diaphragm in the FPR and it wasn't getting the fuel it needed under higher rpm.

 

I agree with the other posts above here  - it seems like you have multiple issues going on at once, and I agree your stuck in closed loop - which should be a CTS problem.