Strange idle issue - Running out of ideas

[Dhalsim]

Need some help with a strange idle issue.

91 NSX with CTSC


Symptoms: 1) cold start issue, car wont idle when cold 2)once the car is warm I can start it and it will idle normal but if I press on the throttle the RPMs will increase and then the car will die.


When I drive the car everything seems to be normal and running fine but car will stall at every stop.


Things fixed/replaced/checked
1. Confirmed MAP sensor is good by switching out a known good one
2. Replaced fuel pump - no change
3. Swapped another CTSC RRFPR in - no change
4. Confirmed fast idle valve working normally
5. Confirmed ICV valve working normally by swapping in known good one
6. Replaced ECT sensor
7. Checked for vacuum leaks with smoke machine - none found
8. Swapped known good ECU - no change
9. Fuel feed line was found pinched - fixed but no change
10. Injectors just serviced
11. Plugs were found to be fouled - replaced with new - no change
12. TPS voltage is at 0.51 volts (spec is 0.45V) Confirmed voltage sweeps to 4.5V at full throttle.
13. vacuum line to manifold is free flowing
14. confirmed wiring to ECT sensor is good and grounded
15. confirmed EGR is not stuck open
16. swapped in known good resister box - no change
17. compression test done - no issue
18. added an extra ground from chassis to motor - no change
19. bypassed CTSC ESM and no change
20. coils confirmed dry and not an issue


Notes:
1. mechanic says car is running very rich
2. oil pressure on cold start is almost max at idle
3. One time while the car was idling after been warm the idle started hunting and then died
4. Car was having cold start issue which I was working on when I realized there was a problem with the CTSC. Sent the CTSC in for all new internals and reinstalled into the car. Cold start issue remained but now there is this strange idle issue. Related to installation of CTSC???


I welcome any ideas on what might be going on here or where/what to look at next.


TIA

 

[jwmelvin]

Sounds like a leak in the bypass hose and/or (to a lesser degree) a dirty throttle body.

 

[Dhalsim]

Sounds like a leak in the bypass hose and/or (to a lesser degree) a dirty throttle body.

Thank you for your reply. I will have the mechanic look at these things.

 

[Old Guy]

You said your mechanic said that the engine is running very rich. How did the mechanic determine this? Has the OEM ECU generated error codes indicating that the AFR is out of range or are you monitoring AFR with wide band O2 sensors or was this based on the assessment of the fouled plugs?

Do you have a cold start issue or an idle issue when cold or both? Overly rich can lead to idle performance issues; however, typically excessively lean is more problematic. Cold starting problems tend to be lean related unless you are over fueling the engine during cranking leading to flooding.

You have checked most of the likely boxes that I would suggest. Given the running rich assessment, the one thing that I did not see is confirmation that the fuel pressure is correct. Excessive fuel pressure will cause rich operation under all conditions.

On point 19, you said you by-passed the CTSC ESM. Is that the equivalent of the AEM FIC and if so, did by-passing the ESM mean that you were running directly off of the OEM ECU fuel maps?

 

[jwmelvin]

On point 19, you said you by-passed the CTSC ESM. Is that the equivalent of the AEM FIC and if so, did by-passing the ESM mean that you were running directly off of the OEM ECU fuel maps?

The ESM simply clamps the MAP sensor voltage at that corresponding with atmospheric pressure so the ECU doesn’t freak out by seeing boost. It should have no effect at idle. Cars set up like this always run off the OEM ECU fuel maps, as modified by the increased fuel pressure in boost of the so-called RRFPR.

 

[Old Guy]

The ESM simply clamps the MAP sensor voltage at that corresponding with atmospheric pressure so the ECU doesn’t freak out by seeing boost. It should have no effect at idle. Cars set up like this always run off the OEM ECU fuel maps, as modified by the increased fuel pressure in boost of the so-called RRFPR.

So, any increased fuel delivery above a MAP of around 100 kPa is strictly a function of the altered reference pressure on the regulator? I guess that sort of works; but, doesn't really provide an opportunity to exactly tune the fuel system. However, as you point out, if the engine is running directly off of the OEM fuel map then its not really pertinent to the current problem. The OP noted that they replaced fuel pressure regulator. Is there a fuel pressure setting for testing the operation of the regulator to confirm that something is not screwing up the operation of the system. About the only other items that I think could screw up fuel mixture during starting are the manifold air temp sensor and the coolant temp sensor (ECU not dash sensor). Checking the sensor resistances and the wiring would be useful. A bunged O2 sensor can mess up fuel mixture; but, that should not cause a starting problem since the engine starts up in open loop with no O2 correction. Once the engine is up to operating temperature it goes into closed loop operation and at that point a faulty O2 sensor could mess things up However, if there was a faulty O2 sensor I would be inclined to think that would normally cause a mixture out of range error code.

 

[Dhalsim]

You said your mechanic said that the engine is running very rich. How did the mechanic determine this? Has the OEM ECU generated error codes indicating that the AFR is out of range or are you monitoring AFR with wide band O2 sensors or was this based on the assessment of the fouled plugs?

Do you have a cold start issue or an idle issue when cold or both? Overly rich can lead to idle performance issues; however, typically excessively lean is more problematic. Cold starting problems tend to be lean related unless you are over fueling the engine during cranking leading to flooding.

You have checked most of the likely boxes that I would suggest. Given the running rich assessment, the one thing that I did not see is confirmation that the fuel pressure is correct. Excessive fuel pressure will cause rich operation under all conditions.

On point 19, you said you by-passed the CTSC ESM. Is that the equivalent of the AEM FIC and if so, did by-passing the ESM mean that you were running directly off of the OEM ECU fuel maps?

The mechanic only stated that he placed a piece of cardboard in front of the exhaust and it was black very quickly. I am unsure if he has set up an AFR gauge on the car. I don't already have an AFR gauge installed.

The car will not idle when cold and has to become fully warm at which point if I take my foot off the gas it dies. But will start right back up and idle just fine until I blip the throttle and then the car will rev up and then the revs will come down to zero and die.

I was having some fuel pressure issue so I purchased a fuel pressure gauge. I had to max out the RRFPR to get the idle pressure to 40psi. This made us think there might be problem with the RRFPR or the fuel pump. We switched out the RRFPR but no change. We replaced the fuel pump but no change.

The next steps we are taking are to clean the throttle body and check the main relay to make sure all the solder points are good.

 

[Jinks]

Bumping this so I subscribe. Roger, have you changed your fuel filter yet?

 

[Dhalsim]

Bumping this so I subscribe. Roger, have you changed your fuel filter yet?

Fuel filter was changed about 2 years ago

 

[Dhalsim]

Car is fixed!

It was one of 2 things that fixed it. 1) The mechanic did a valve adjustment. He stated that valves were out of spec but nothing unusual 2) when he pulled the SC yesterday he noticed some fuel on top off one of the closed valves (as he had already said it was running super rich). He also noticed that the fuel rail spacer was not on and found it lying on the intake manifold. He reinstalled this and put the SC back on. His theory is that possibly one of the injectors was not seated correctly due to the spacer not been on and possibly air was getting past the injector and into the engine making the O2 sensor think the car was running lean and adding too much fuel thereby stalling the engine.

 

[solidol]

Glad you got it solved. Out of spec valves won't cause symptoms you were describing. Not properly sealed injector definitely will. Wonder who installed the fuel rail [emoji12]

 

[jwmelvin]

That’s great you figured it out. Kind of amazing the spacer was just sitting there. Thanks for the report.

 

[Valhalla]

Wow, very lucky you found this. No smell of raw fuel? I would have been scared to death if it had been spraying fuel under your SC. Glad you got it fixed.