7k rpm rev limit and no VTEC

[coreys]

My 1991 5 speed NSX is being rev limited around 7k rpm and I do not hear the VTEC sound kicking in any more. I have aftermarket headers and exhaust, so the VTEC is usually easy to hear. It does not have a check engine light and the light does still work. I read that this might be caused by one of the coolant temperature sensors, but my idle does not seem to be affected. It idles fine below 1k rpm after a few minutes. Any idea what is causing the premature redline and VTEC not engaging?

Thanks!

 

[docjohn]

how is your oil level?

 

[coreys]

The oil is at the top mark on the dipstick. It's had maybe 500 miles since it was changed last, so it doesn't appear to be leaking / eating any.

 

[SWFL_NSX]

FSM has a vtec trouble shooting flow chart starting on section 6-15.

Looks like there are only 4 inputs-rpm, load, vehicle speed, and water temp. Diagnostic procedures take you through troubleshooting the oil pressure switch and the valve itself.

I've heard that the engine has a rev limiter at 6800rpm before the engine is fully warmed up. Might want to start by figuring out if there really is a limiter, and if it runs from the same sensor as the one that goes to vtec.

EDIT: Check out post 10 on the following thread-Kaz confirms a 7k rpm limit as well as vtec disengagement if the TW sensor is reading below a certain amount. Keep in mind there are 3 different temp sensors.

http://www.nsxprime.com/forum/showthread.php/199019-7-800rpm-rev-limit?highlight=tw+sensor

 

[twentyfourth]

Had same/similar problem- no check engine lights. Was a vtec solenoid. Part was maybe nla but ordered one from someplace in California. Hth- ira

 

[coreys]

A problem with the VTEC solenoid makes sense. My coolant expansion tank sprayed out of one of the seams and coolant got right on the front VTEC solenoid about the same time it quit working. That makes me think it's something electronic on the solenoid. Would I get the same symptoms if it was just the oil pressure switch not working on that solenoid?

 

[Old Guy]

There are four ECU trouble codes related to the VTEC system

21 - front solenoid
22 - front oil pressure switch
51 - rear solenoid
52 - rear oil pressure switch

The service manual is not explicit about this; but, my speculation is that if the VTEC solenoid goes open circuit (failed) then the ECU will generate a 21 or 51 code. If the ECU generates a control signal to the VTEC solenoid and the oil pressure switch (normally closed) does not go open (lack of oil pressure) then I am also speculating that the ECU generates a 22 or 52 code. Since you are not generating any codes (did you use the service check connector to look for stored codes?) the VTEC solenoids MIGHT be OK.

So two possible things to consider:

First
The ECU will probably not generate VTEC error codes if the conditions to engage VTED that @SWFL_NSX mentioned are not being met
Temp > 60C
RPM>5800
Speed > 5 km/hr
Load ????
I would assign a higher probability to the possibility that coolant sensor has gone out of spec or that a bad connection at the sensor is tricking the ECU into reading a low coolant temperature. The FSM test procedure is to run the engine up to normal operating temperature, stop the engine and measure the resistance of the sensor. If the sensor resistance is between 200 and 400 ohms then the sensor is probably OK and you should be meeting the temperature requirements for VTEC operation. Check the wiring at the plug to make sure that it is OK. It is possible that the VSS might be causing a problem (any codes). A problem with the RPM signal or the load signal would likely render the engine in operable so those are likely not the cause of the problem.

Second
It is possible that everything is working just fine with the ECU and the VTEC solenoids and the oil pressure switches are working as designed. However, you are not getting enough oil pressure to activate the pistons in the rockers. There are screens / filters at the interface between the solenoid assembly and the cylinder head. If these get gummed up, they can result in loss of pressure and failure to activate the pistons in the rockers. Sticking of the spool valves might result in similar loss of oil pressure. Without looking at the physical arrangement of the VTEC solenoid assembly or knowing the set points on the switches, I don't know whether pressure loss caused by a plugged screen or sticky spool valve would cause a 22 or 52 error code. There are threads on Prime discussing the cleaning / replacement of the VTEC screens and cleaning of the VTEC solenoids.

In summary:
It would seem odd that both VTEC solenoids would run into low pressure problems all of a sudden. I would expect a clogged oil screen problem to have some prior symptoms and not show up on both solenoids at exactly the same time. As such, I would put a lower probability on the solenoids as a cause. The fact that this is accompanied by RPM limiting (is it a hard fuel cut?) makes me think that it is likely that the input conditions for VTEC operation are not being met. Check the temperature sensor and the VSS just because it is probably easier to rule those out first. If the temperature sensor and VSS are OK, then you have no choice but to get into the VTEC solenoids. Electrical tests and checking connections on the solenoids is not too bad; however, I would expect electrical problems to result in error codes. All the rest of the solenoid / spool valve inspection is definitely more work, messier (the oil pressure test) and potentially more expensive.

 

[coreys]

I did not check for stored codes. I always assumed there were no codes unless the CEL is on. Should I check it? I have never seen the CEL on, but the light does work when I first start the car.

It doesn't feel like a hard fuel cut, it's more like a standard rev limiter where it just bounces at 7k rpm (reducing timing rather than cutting fuel?).

It sounds like I should test the coolant sensor. I know there are multiple coolant sensors. I'm assuming you are talking about the TW Sensor that is right below the front VTEC solenoid? That sensor was also probably splashed with coolant around the same time my problems started.

 

[Old Guy]

Some errors have to reoccur a few times before the MIL lights up. I do not know whether the VTEC codes fall into that category. Since you are a new Prime member, is the car newish to you? If so, checking for stored codes is easy to do, no cost and it may shed some useful light on the problem (perhaps a VSS problem?). However, if the battery has been disconnected, then any stored codes will have been erased.

I can't comment on the nature of the cut that may be associated with VTEC. I could not find anything in the service manual that describes a temperature related RPM limit. The hard cut for the 8300 RPM rev limit is a fuel cut which resumes after the engine speed drops about 200 RPM so you can get this on - off kind of response where fuel cuts, then resumes and then cuts and then resumes and then cuts .... if you stay at the limit.

Yes. The coolant sensor for the ECU is on the left front side of the engine (but so is the gauge sensor). Not exactly the easiest thing to find. Check out post #3 from Kaz with a picture showing the location for Tw.

http://www.nsxprime.com/forum/showt...erature-reading-high?highlight=coolant+sensor

I can feel my back muscles going into spasm trying to get at that sucker! If the sensor reading does not fall within spec on a hot engine, before you change it make sure that the engine coolant temperature is actually coming up to the specified operating temperature. If you lost coolant because of a split coolant container and that was enough to cause overheating of the thermostat, then you might have damaged the thermostat. In my limited experience (not on my NSX), every time I have had an overheating event, my thermostats go open and tend to stay open resulting in the engine not coming up to temperature. I don't know whether the NSX thermostat does that; but, worth checking. The NSX thermostat also seems to have a bit of a reputation for developing by-pass leaks which results in the engine not coming up to temperature or taking forever to come up to temperature when the outside air temperature is low.

 

[nsxspdfreak]

Do check for codes. Don't rely on your hearing to determine if Vtec is engaging. The ECM limiting rpm may have nothing to do with Vtec at all. If you want to check to see of the solenoids and CEL are operating the easiest way to do is to disconnect the wire connectors to each solenoid and then run the engine up to vtec range. Once it hits vtec range you'll get a check engine light and all 4 failure codes (if you disconnect both at the same time) and the ECM will limit the rpms. You'll have to pull the clock fuse to reset. If don't get any codes you have something going on with the ECM. I've also run 12v test lights connected to the solenoids that I could watch in the cabin while I was driving.

 

[Okami-san]

Sorry to raise an old thread, but this is the newest thread I could find that slightly applies.

I have a 1991 Manual. 166k on the Odo. Smell of burning oil when driving. Turns out all 4 cam plugs, the oil pan, and something around the VTEC Solenoids is leaking.

All but the solenoid leak is easy to see. But where on the solenoid leak is troublesome to determine (in the car, in my garage).

Since I need cam plugs also, should I just drop the engine and find some way to pressurize the solenoids to find the leak? If it is the body of the solenoid (as the mechanic suggested), there's no longer parts available either. If it is just the gasket, then no big deal, since they are still available.

Any help on diagnosis and repair would be helpful.

Thanks in advance

 

[SWFL_NSX]

All but the solenoid leak is easy to see. But where on the solenoid leak is troublesome to determine (in the car, in my garage).

Leaking from the gasket between the solenoid and the engine block is very common; leaking from between the base and the solenoid itself it common as well...this is probably what your mechanic saw when he said "leaking from the body." Leaking from the plug-to-solenoid seal is rare. All gaskets are available, please replace all bolts as well as the solenoid-to-block bolts seem to corrode over time.

This thread is a good overview of what to order and how to replace the gaskets. http://www.nsxprime.com/forum/showt...-through-(pics)/page3?highlight=15825-PR3-005

Here is a parts diagram that shows the gaskets: https://www.oemacuraparts.com/v-1992-acura-nsx-coupe--nsx--5-speed-manual/engine--spool-valve

Since I need cam plugs also, should I just drop the engine and find some way to pressurize the solenoids to find the leak? If it is the body of the solenoid (as the mechanic suggested), there's no longer parts available either. If it is just the gasket, then no big deal, since they are still available.

Dropping the engine is pretty involved. I would replace the gaskets for the solenoids and oil pressure sensor, clean the area thoroughly and make very sure the leak isn't from the valve covers before considering dropping the engine.

Depending on how the car is used (daily driver? show car?) you might put off fixing minor oil seepage until a major service like clutch or timing belt replacement. Other things should be addressed at the same time as an engine out service as well-LMAs, coolant hoses, etc.

 

[Okami-san]

Thanks for the fast response.

I'm wondering if the mechanic is thinking that the middle gasket (held by the red dot bolts in the first link) are the "body" leak. I think I may just order the gaskets and give it a whirl.

But I do still have the cam plugs and oil pan to figure out. Maybe wait until winter when I don't drive it anyway.

Thanks again.

Leaking from the gasket between the solenoid and the engine block is very common; leaking from between the base and the solenoid itself it common as well...this is probably what your mechanic saw when he said "leaking from the body." Leaking from the plug-to-solenoid seal is rare. All gaskets are available, please replace all bolts as well as the solenoid-to-block bolts seem to corrode over time.

This thread is a good overview of what to order and how to replace the gaskets. http://www.nsxprime.com/forum/showt...-through-(pics)/page3?highlight=15825-PR3-005

Here is a parts diagram that shows the gaskets: https://www.oemacuraparts.com/v-1992-acura-nsx-coupe--nsx--5-speed-manual/engine--spool-valve



Dropping the engine is pretty involved. I would replace the gaskets for the solenoids and oil pressure sensor, clean the area thoroughly and make very sure the leak isn't from the valve covers before considering dropping the engine.

Depending on how the car is used (daily driver? show car?) you might put off fixing minor oil seepage until a major service like clutch or timing belt replacement. Other things should be addressed at the same time as an engine out service as well-LMAs, coolant hoses, etc.