My 94 is now idling like a champ and runs with strong power, but. It seems to have a stumble at idle when placed in drive. Its almost harmonic. It does this at 850 rpm. In park, it does not do this. I've been told that our cars hate to idle. Is this common? Aside from this I am super happy after: Spark plug change, Upgrading the Throttle body, all fluid changes, all temp sensors changed, cleaned out most of the intake manifold, serviced the injectors (which 4 of 6 were dripping), changed pcv valve, changed fuel filter, cleaned Iac valve. Did I miss anything you guys would change. Looking to have the valves adjusted soon.
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For the minority with automatics......
- Thread starter El-Cojonico
- Start date
I don't think the C30 / C32 engines hate to idle. My C32 idles just fine at a very steady 850 ish RPM when hot. A pure 90 deg V6 does have an uneven firing order. The common design approach to deal with this is to put offsets into the crank pins to reduce the unevenness. It does not eliminate it and I notice that when cold my engine has a slightly manic pounding / loping 'sound' that is very noticeable in the cabin. Not really noticeable if you are outside the car and listening to the engine with the engine cover up. I have no empirical evidence; but, I suspect that this might be related to the stiffness of the front engine mount when cold and transmission of noise into the cabin. Once stuff is hot, none of this is noticeable. I do know from others that if the front engine mount fails you will get lots of engine noise in the cabin. Stiffer after market mounts also increase the engine noise in the cabin.
Stumble at idle when placed in drive? Does that mean an engine hesitation when you first press the throttle pedal down or does that mean that with the car in drive; but stopped at a traffic light pressing on the brake pedal, the idle is problematic?
- if the first, when you upgraded the throttle body, what did you do with the throttle position sensor? Is it new and properly adjusted? If it is original, was its operation checked? The engine gets most of its operating information from the MAP sensor and will operate 'mostly OK' even with the throttle position sensor disconnected (if you ignore the MIL light). One of the refinements that the TPS provides is that it tells the ECU that the driver has just pressed down on the throttle pedal and that the ECU needs to provide a tiny shot of fuel (commonly called X-tau enrichment) to maintain smooth operation. Its the equivalent of the accelerator pump on a carb equipped car. If your TPS is worn or slightly out of adjustment the ECU may not be getting this signal leading to a hesitation when you first press down on the throttle. The procedure for checking and adjusting the TPS is in the service manual.
- if the second describes the symptom, then I am slightly out of my league. However, I do know that the transmission control module gets a signal from the brake pedal switch and a vehicle and engine speed signal and the transmission control module communicates with the ECU. Presumably, with the shifter in drive; but, the brake pedal pressed down and the vehicle speed at zero and the engine speed below some value (car at a stop light) I suspect the module tells the ECU to put the engine into idle control mode. If the control module is not getting the brake signal it might not be going into idle control mode. If one of the other two signals is missing, I think you would get a DTC; but, not with a brake signal switch. So, perhaps check to make sure that the transmission module is getting the brake signal and use the service check connector to see if there are any stored trouble codes.
Stumble at idle when placed in drive? Does that mean an engine hesitation when you first press the throttle pedal down or does that mean that with the car in drive; but stopped at a traffic light pressing on the brake pedal, the idle is problematic?
- if the first, when you upgraded the throttle body, what did you do with the throttle position sensor? Is it new and properly adjusted? If it is original, was its operation checked? The engine gets most of its operating information from the MAP sensor and will operate 'mostly OK' even with the throttle position sensor disconnected (if you ignore the MIL light). One of the refinements that the TPS provides is that it tells the ECU that the driver has just pressed down on the throttle pedal and that the ECU needs to provide a tiny shot of fuel (commonly called X-tau enrichment) to maintain smooth operation. Its the equivalent of the accelerator pump on a carb equipped car. If your TPS is worn or slightly out of adjustment the ECU may not be getting this signal leading to a hesitation when you first press down on the throttle. The procedure for checking and adjusting the TPS is in the service manual.
- if the second describes the symptom, then I am slightly out of my league. However, I do know that the transmission control module gets a signal from the brake pedal switch and a vehicle and engine speed signal and the transmission control module communicates with the ECU. Presumably, with the shifter in drive; but, the brake pedal pressed down and the vehicle speed at zero and the engine speed below some value (car at a stop light) I suspect the module tells the ECU to put the engine into idle control mode. If the control module is not getting the brake signal it might not be going into idle control mode. If one of the other two signals is missing, I think you would get a DTC; but, not with a brake signal switch. So, perhaps check to make sure that the transmission module is getting the brake signal and use the service check connector to see if there are any stored trouble codes.
Old Guy, you are awesome. If I pulled you over I'd let you off with a warning.
The second is what describes my issue more accurately. Its annoying for the car to run perfect, then you get to a light and it feels like a melodic bump and I shift into neutral to avoid it. I guess I really need to take it to a pro.
Do you think a Valve Adjustment would help?
The second is what describes my issue more accurately. Its annoying for the car to run perfect, then you get to a light and it feels like a melodic bump and I shift into neutral to avoid it. I guess I really need to take it to a pro.
Do you think a Valve Adjustment would help?
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If they are not obviously noisy, I would be inclined to leave them until such time as the timing belt is due for maintenance. And, if they have not been replaced already, you would probably want to plan to for upgrading the LMAs with something like the SOS kit. One of the risks associated with just doing the valve adjustment is that if you disturb gaskets that are original and if you put it back together with those gaskets you could end up triggering a slow oil leek. You could do a gasket replacement when you do the valve check; but, then do you replace the gaskets again when you do the timing belt??? If a timing belt replacement is in the near future, I would be inclined to wait until its time for the belt. If the valves are noisy, then check the clearances (and perhaps advance the timing belt replacement so its all done at once?).
If the car idles nicely when it is shifted into neutral, I don't think valve clearance adjustment is going to fix the problem (even if the clearances are incorrect). I think the problem is related to the transmission controller not getting the brake signal telling it to go into some kind of idle control mode.
If the car idles nicely when it is shifted into neutral, I don't think valve clearance adjustment is going to fix the problem (even if the clearances are incorrect). I think the problem is related to the transmission controller not getting the brake signal telling it to go into some kind of idle control mode.
Could it have to do with a load (In drive) and being at such a low RPM when stopped at a red light. Gonna get the coil paks checked. I'm a preventative maintenance freak, so any suggestions are welcomed especially at this high mileage.
Valve adjustments are just not really needed on the NSX. If you must: wait for the timing belt change.
Rather get a compression test...that will show any valve issues without a lot of work. Also gives you a chance to inspect plugs and coils.
While likely not related: The front mount does go bad first.
Rather get a compression test...that will show any valve issues without a lot of work. Also gives you a chance to inspect plugs and coils.
While likely not related: The front mount does go bad first.
A well-sorted automatic should be able to idle effortlessly down to 500 rpm. I'm not sure I understand the issue you are describing, but it seems like the car idles smoothly in Park or Neutral, but "stumbles" when you shift into Drive. The OBD-I idle control system on the 1994 NSX works through the EACV, which is a solenoid-activated air bypass valve that opens based on input from the ECU and its related systems. When you shift into Drive, you connect the engine to the torque converter, which introduces a load on the engine. To maintain a stable idle, the transmission computer should send a signal to the ECU via the shift position switch sensor, and the ECU should then command the EACV to open more to compensate for the load. The first thing you should do is test the shift position sensor signal to make sure the ECU is getting the right information. The procedure is on pages 11-76 to 77 of the service manual.
If you have a good shift position signal, the next step is to test the EACV. The procedure for that is on page 11-70 of the service manual. Assuming it checks out ok, I recommend removing the EACV from the intake manifold and thoroughly cleaning it. I always replace the filter screen and o-ring and clean out the valve body with throttle body cleaner (make sure to tilt the valve so that the cleaner drains AWAY from the internal coil).
If these steps don't fix the issue, I would make sure your idle is set correctly and your idle screw and passage are perfectly clean. After removing the throttle body, remove the idle set screw, counting the number of turns. Thoroughly clean out the idle air passage with carb cleaner and a non-marring bristle brush. I replace the idle air screw with a new one from Honda, since it is cheap (like 12 bucks), it is pre-coated with the factory red sealant and comes with a new O-ring. The part number is 16016-PM6-005. Install the screw the same number of turns as the old one and then go through the idle set process, which is on page 11-84 of the service manual. Do not rely on the tach in the gauge cluster- you should use a tachometer connected directly to the pickup in the engine bay. Per Honda's spec, the automatics should idle at 600 rpm (+ or - 50 rpm) in no-load condition, which means that the car should be in Park and none of the following items should be operating: headlights, blower fan, rear defogger, radiator fan and A/C compressor. I just shut off the climate control for the test and make sure I measure rpm when the rad fans aren't running, since the car will be warmed up. Once you get a good, solid idle, seal the idle screw in place with some clear silicone.
Hope this helps.
I'll try your recommendations this weekend. I had a work friend come over, who is a Honda tech. Without much investigating he thinks the engine is running exceptionally smooth. He attributes the minor stumbling to an aftermarket Crank Pulley and the upgraded throttle body. I described the tons of carbon I cleaned out of the top of the manifold when I set my injectors for service and he believes I might have some minor blockage in the EGR valve, just not enough to set off a CEL. He also told me that with 132k miles, my transmission is very worn and that is also a contributing factor. I guess I'm just a perfectionist.
I don't think its your EGR. The EGR valve should be closed at idle. If a piece of crud is holding the valve open this will allow exhaust into the intake at idle, screwing up the fuel mixture and confuses the O2 sensors when in closed loop and can mess with your idle. However, EGR leakage would cause problems for idle whether the car is in park, neutral or stopped with the brake applied. Since the car idles just fine in park or neutral, I am not buying the EGR cause.
I also don't buy the upgraded throttle body as a cause - unless it is somehow selectively messing up the EACV operation. If the throttle body was the cause, again it would be a common factor in park, neutral or stopped with the brake applied.
Honcho is correct about additional load being applied on the engine with the car in drive; but, the brakes applied. The ECU needs to apply idle-up to the EACV to keep the engine from stalling under this increased load condition. This idle-up is similar to the idle-up it applies when you turn on the air conditioner. In fact, I suggest that as a test. With the car in neutral or park, switch on the air conditioner and see if the idle-up function engages. If the idle-up function works for the air conditioning, I offer up the opinion that the EACV may be carrying out its function OK and that the problem is not caused by a dirty EACV. However, on a 1994 car cleaning the EACV is probably a good preventative maintenance task and it eliminates the EACV as a factor in your problem. If the idle-up does not work for the air conditioning, then EACV cleaning is definitely something to tick off. In order to apply idle-up when stopped in drive with the brakes applied, the ECU needs to be informed that the conditions are correct to apply idle-up, specifically that the car has come to a stop or is below some threshold speed, the throttle is closed, the transmission is in drive and the brakes are applied. The ECU and the transmission controller get all of those signals; but, I don't know whether it uses all of them or just one or two. The shift position function appears to be working (it idles in neutral and drive) which is why I come back to beaking about confirming the operation of the brake input to the transmission controller.
I also don't buy the upgraded throttle body as a cause - unless it is somehow selectively messing up the EACV operation. If the throttle body was the cause, again it would be a common factor in park, neutral or stopped with the brake applied.
Honcho is correct about additional load being applied on the engine with the car in drive; but, the brakes applied. The ECU needs to apply idle-up to the EACV to keep the engine from stalling under this increased load condition. This idle-up is similar to the idle-up it applies when you turn on the air conditioner. In fact, I suggest that as a test. With the car in neutral or park, switch on the air conditioner and see if the idle-up function engages. If the idle-up function works for the air conditioning, I offer up the opinion that the EACV may be carrying out its function OK and that the problem is not caused by a dirty EACV. However, on a 1994 car cleaning the EACV is probably a good preventative maintenance task and it eliminates the EACV as a factor in your problem. If the idle-up does not work for the air conditioning, then EACV cleaning is definitely something to tick off. In order to apply idle-up when stopped in drive with the brakes applied, the ECU needs to be informed that the conditions are correct to apply idle-up, specifically that the car has come to a stop or is below some threshold speed, the throttle is closed, the transmission is in drive and the brakes are applied. The ECU and the transmission controller get all of those signals; but, I don't know whether it uses all of them or just one or two. The shift position function appears to be working (it idles in neutral and drive) which is why I come back to beaking about confirming the operation of the brake input to the transmission controller.
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Just for $hits and giggles I will be replacing the fuel pump next weekend. It may have not failed but may be weak from 25 years and 132k miles. I'll let you guys know the outcome.
Changed fuel pump to a Walbro….No change in idle issue.
Success...….It was the rear coil packs!!!! They were corroded so I replaced all 3 and she is running and idling PERFECTLY. Took her for a test spin and I'm back in love. Thanks to all who helped me out!!!
Can't argue with success; but, that is bizarre. Its hard to reconcile the coils causing a problem at idle; but, working just fine under load.
Old Guy, It may have been that the miss was minor on acceleration and I didn't notice it. But it was apparent at idle, especially in Drive and stopped at a red light.....almost embarrassing. But I'm thankful for you guys and your knowledge. I've had other bad ass cars, but this one is new to me. Now I'm not afraid to tackle anything on it. This website rules!
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