I have a 91 NSX and at some point I started to hear a popping noise out of the tail pipes. Used to happen every once in a while and disappear when I kicked the revs up, but now it's progressed and seems to happen all the time. Any insights and advice are appreciated.
exhaust popping
- Thread starter brad73
- Start date
Popping on rapid de acceleration is typically caused by unburnt fuel making its way through the combustion chamber and then exploding in the header / manifold when it comes into contact with O2 and hot metal. Quite common on old school over carburetted engines. Most fuel injected engines have a fuel cut function on de acceleration which is enabled by the ECU detecting a fully closed throttle plate which shuts off the fuel until the RPM drops to something like 1000 - 1200 RPM at which point fuel flow is re established (or you press down on the throttle which re establishes fuel and air flow to re accelerate the engine which is consistent with your description of 'kicked the revs up'). If the fuel cut function is not working, this can result in fuel entering the engine during de acceleration which is not burnt because of lack of O2 (closed throttle) which then results in an explosion in the manifold. Even this circumstance should be rare because the presence of catalytic converters and an OEM muffler make the chance of atmospheric O2 working its way back up the exhaust system to combust in the header pretty remote. If you have removed your cats and have a free flow exhaust system then welcome to 'pop city'. The other thing that can induce popping is a leak in the exhaust system between the head (flange gasket) and the catalytic converter. This will allow atmospheric O2 to enter the exhaust system and mix with unburnt gas resulting in the pop.
Note that some cars do not do a 100 % fuel cut on closed throttle de acceleration. They may do something like an 80 - 90% fuel cut because the 100% cut can make for a very jerky transition when you re apply the throttle. This probably makes them more susceptible to popping on de acceleration. I don't know how the 1991 NSX ECU is set up.
There may be other contributing factors; but, I suggest that you check the operation of your throttle position sensor as described in the service manual paying particular attention to the voltage / resistance at the closed setting. The other thing to check is for leaks in your exhaust system.
I have never heard of exhaust popping under steady load or acceleration. If you are hearing popping under steady load or acceleration are you sure that you are not experiencing misfires? Dumping enough fuel into the engine under load that unburnt fuel is exiting into the header to explode should trigger an MIL light because the mixture is out of range.
Note that some cars do not do a 100 % fuel cut on closed throttle de acceleration. They may do something like an 80 - 90% fuel cut because the 100% cut can make for a very jerky transition when you re apply the throttle. This probably makes them more susceptible to popping on de acceleration. I don't know how the 1991 NSX ECU is set up.
There may be other contributing factors; but, I suggest that you check the operation of your throttle position sensor as described in the service manual paying particular attention to the voltage / resistance at the closed setting. The other thing to check is for leaks in your exhaust system.
I have never heard of exhaust popping under steady load or acceleration. If you are hearing popping under steady load or acceleration are you sure that you are not experiencing misfires? Dumping enough fuel into the engine under load that unburnt fuel is exiting into the header to explode should trigger an MIL light because the mixture is out of range.
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Completely agree. Another thing to check is the fuel injectors could be dirty. I have seen this happen, especially when a car sits no used for a while. The fuel injectors can get dirty(old fuel), and in fact they can drip, even when closed.
HTH,
LarryB
Thanks old guy. The muffler is off and it's possible the cats are bad. I don't drive it much so I use fuel additives to keep the gas from going bad, and I heard they're not so good for the converters. Meanwhile, I'll look into the throttle position sensor issue.
Thanks. I had a good feeling about old guy.
If the car has been stored for a long time with gas in the injectors, I would be inclined to pull them out and send them to RC Engineering (rcfuelinjection.com) or Witch Hunter Performance for cleaning and testing (that will confirm that they don't leak and have equivalent flow rates and offsets). As Larry B. notes, a drooling injector would definitely contribute to popping on trailing throttle. If you pull the injectors for cleaning, order new seal rings (4 in total I think for each injector - others can confirm). If you have a local injector service that you are confident in, you can go with them; but, RC and Witch hunter are known to be reliable services and do a lot of mail order work.
In fact, given that bit of non-service history about the fuel system I would be inclined to clean the injectors to see if that helps the problem before touching the throttle position switch.
The 'muffler is off' would increase the chance of experiencing exhaust popping. Bad cats would not necessarily increase the problem unless bad cats meant that the core structure of the cats had disintegrated and blown out the tail pipe in which case the cat is effectively an empty shell - that would increase the chance of exhaust popping. I would be inclined to do an inspection (difficult on the NSX) and a listen test (normally easy - not so easy if your muffler is off :smile
to try and confirm that you do not have any leaks at the manifold flange gasket. Its probably unlikely; but, aside from contributing to the popping a leak at the flange gasket can be bad for the exhaust valves under the right circumstances.
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