engine bog around 3000 rpm

[nsxbaby]

I have a 91 with 190k on it. i am having an issue with it bogging down around 2500 to 3500 rpm then it will pick up and blast off fine. It doesn't do it all the time but it is more frequent lately. When it happens the exhaust note will change as I apply more throttle it doesn't really pick up speed then suddenly at 4000 rpm its like normal. After maybe an hr of this it will throw the CEL and its all good. i have new coils, plugs and clean throttle body. I have removed the clock fuse to clear any old codes but couldn't get it turn the CEL back on during a 100 mile trip even though it was doing the same thing after about 50 miles.
I have tried the search but it's kinda wordy to but into the search in way to sort the results. i think this problem is somewhat common and sometimes directed to water in the plug /coil pack but I am sure this is not the case. I still have the original o2 sensors and injectors on the car. If anyone has any suggestions until I can get a code out of it would help.

 

[RYU]

Some areas I would look into.

* Change the fuel filter - might be clogging up
* Send your injectors to RC Engineering for a test and clean
* Check your grounds (some get corroded) especially the one by the ignitor box.

There are several others as you head down this rabbit hole but i'd start with these.

 

[goldNSX]

Fuel delivery or fuel mixture.
Fuel filter (change) or fuel pump resistor (bypass) or fuel pump (change) itself.
o2-sensors de-connect the plugs which will throw a CEL but if the bogging is away it's the o2-sensors.
As you mention that it's all ok above 4k rpm I suspect the fuel pump resistor or the fuel pump due to its age. Very, very common faults.

 

[Old Guy]

Because of the transition RPM, I am also thinking a fuel pump or resistor issue. That RPM is close to the transition where the ECU shunts the resistor applying full voltage to the fuel pump. The resistor might be faulty (out of spec due to a bad condition); but, at the age of your car a tired fuel pump is a very real possibility. It may not be capable of delivering specified pressure with the resistor dropping the voltage. When the resistor is shunted full voltage is applied to the pump allowing it to develop a little more pressure. A fuel pressure test would be appropriate before fiddling with the pump, resistor or the fuel filter; but, the paper clip in the resistor is an easy test to do.

The error code will assist in diagnosis. If you get fuel mixture out of range error code(s) low fuel pressure could be the problem.

 

[Honcho]

Fuel delivery or fuel mixture.
Fuel filter (change) or fuel pump resistor (bypass) or fuel pump (change) itself.
o2-sensors de-connect the plugs which will throw a CEL but if the bogging is away it's the o2-sensors.
As you mention that it's all ok above 4k rpm I suspect the fuel pump resistor or the fuel pump due to its age. Very, very common faults.

Because of the transition RPM, I am also thinking a fuel pump or resistor issue.

That makes three of us. Based on age and mileage, it's likely the pump and/or resistor are bad.

 

[goldNSX]

Regarding the high miles of 190k I'd recommend to plan the change of the fuel pump anyway. It's a consumable as the plastic impellers do wear out with miles. Just a preventative repair before it fails definitly. For the cost-sensitive owners rockauto.com has more than 10 different fuel pumps and even a Denso 9510011 for around $80 which would be my choice but I've never tried it out.

 

[asylum]

After maybe an hr of this it will throw the CEL and its all good.

If everything is ok with the CEL light on, I really suspect your O2 sensors. When CEL is on, ECU goes into a default mode than does not take account of the O2 sensors to calculate the amount of fuel to inject and your car runs fine (with bad mpg thought).

Because of the transition RPM, I am also thinking a fuel pump or resistor issue. That RPM is close to the transition where the ECU shunts the resistor applying full voltage to the fuel pump. The resistor might be faulty (out of spec due to a bad condition); but, at the age of your car a tired fuel pump is a very real possibility. It may not be capable of delivering specified pressure with the resistor dropping the voltage. When the resistor is shunted full voltage is applied to the pump allowing it to develop a little more pressure. A fuel pressure test would be appropriate before fiddling with the pump, resistor or the fuel filter; but, the paper clip in the resistor is an easy test to do.

The error code will assist in diagnosis. If you get fuel mixture out of range error code(s) low fuel pressure could be the problem.

I might be wrong, but even with the CEL light on, the problem would remain if it is a pump/resistor/fuel filter problem. Nsxbaby says it is all good with the CEL on.

 

[nsxbaby]

I am thinking like asylum is saying all the other possibility's are mechanical and I would think the symptoms would remain when the CEL is on. Once the light comes on the problem is gone.
I am going to try and drive it this weekend to see if I can get it to throw a code.

 

[JohnWayne]

I am thinking like asylum is saying all the other possibility's are mechanical and I would think the symptoms would remain when the CEL is on. Once the light comes on the problem is gone.
I am going to try and drive it this weekend to see if I can get it to throw a code.

Drive it over to Asheville and lets golf! LOL.
Good luck figuring out whats going on

 

[asylum]

I am thinking like asylum is saying all the other possibility's are mechanical and I would think the symptoms would remain when the CEL is on. Once the light comes on the problem is gone.
I am going to try and drive it this weekend to see if I can get it to throw a code.

CEL codes are sometime not very explicit. Disconnect the O2 sensors. It will throw a CEL. But if your car drives fine --> change your O2 sensors (and check their wirings).

 

[Old Guy]

At 190,000 miles, the O2 sensors could be contributing to the problem; but, if it was an O2 sensor problem it would not have a transition in operation around 4000 RPM. O2 sensors that are reading incorrectly will cause operational problems over the whole RPM range up to the point that the ECU ignores them at high engine RPM / load.

After maybe an hr of this it will throw the CEL and its all good. i have new coils, plugs and clean throttle body. I have removed the clock fuse to clear any old codes but couldn't get it turn the CEL back on during a 100 mile trip even though it was doing the same thing after about 50 miles.

After resetting the ECU, low fuel pressure will cause the ECU to accumulate long term fuel trim as the ECU tries to correct the fuel mixture. At the point the accumulated long term trim reaches the 20% value it will trigger the CEL. Depending on how low the pressure is this may occur quickly or take some time before the CEL comes on again.

The fuel pressure check is fairly simple if you have the tools. Do it first since it will definitively indicate whether you have a fuel pump problem. If the fuel pressure tests out fine, then move on to analyzing the O2 sensors as a potential problem; but, the 4000 RPM transition symptom does not really jive with an O2 sensor problem.

As an observation, I am a bit like goldNSX. On a 29 year old car with 190,000 miles and an original fuel pump I would not feel too bad about replacing the fuel pump as a precautionary measure. Having to drop the tank is a hassle; but, probably less of a hassle than trying to extract 29 year old O2 sensors from the exhaust manifold - particularly if they snap off and you have to remove the manifold to extract them.

 

[nsxbaby]

oh no the plot thickens.

 

[goldNSX]

The easiest and 1st test in this scenario is to bypass the fuel resistor and report back. If the car is ok then you can rule out most of the rest.
If you feel more torque at WOT below 4k rpm you have found it.
Having to drop a nearly empty fuel tank is a little bit of work but duable while having to deal with a full fuel tank is no fun.

 

[asylum]

Proceed as suggested.
1) unplug 02 sensors. If your car runs fine, change them. If not,...
2) bypass fuel pump resistor. If car runs fine or better, there is something wrong with fuel pump, filter or resistor.

 

[nsxbaby]

Would I unplug the o2 sensor before it has the CEL or after ? I would think as soon as sensor is unplugged it would go into the closed loop and run fine on the preset program. am I thinking wrong?

 

[goldNSX]

If you unplug the o2 sensors...
- the ECU goes into a 'standard' mode which is a little richer as it can't rely on the o2 sensors. If it runs much smoother now then one or both o2-sensors are giving the ECU wrong info during part throttle operation. At full throttle the 02-sensors are not active. If it doesn't run smoother then there's at least one more problem.
- it will fire the CEL for sure but the code has to be corresponding.
If the CEL lights up just read it out before doing any other test. The code will tell you.

 

[nsxbaby]

Update.
Unplugged o2 sensors and it ran great for appx 100 miles. Reconnected o2 and drove it. After 30 miles problem occurred pulled over and jumped the resistor then continued driveling. Problem reoccurred. So I think it’s the o2 sensors.
By the way the problem seems to occur after a few runs at WOT then drop back down to 3 k range.

 

[Old Guy]

A question for @nsxbaby. Did you ever retrieve the error codes stored in the ECU when the CEL came on or did you just keep resetting the ECU without retrieving the codes? If you have retrieved the error codes that would provide a better indication as to what is going on. If the error codes are 1, 2, 41 or 42 i then it is likely that you have an O2 sensor problem or a wiring problem to the sensor. However, if you have codes 1, 2 , 41 or 42 they should show up almost immediately after doing an ECU reset.

If the error codes are 43, 44, 45, or 46 the diagnosis is more complex. The 1991 service manual is enormously un helpful in defining what triggers these codes; but, my guess from the service manual is that 45 and 46 are the fuel mixture out of range codes triggered by excessive fuel trim and 43 and 44 are ??? (I don't know). The service manual lists O2 sensor problems as the first thing to check for 43 and 44 so it may trigger on a no 14.7 AFR crossing error; but, it also lists fuel system problems as possible causes. For codes 45 and 46 the service manual lists problems in the fuel supply system as the primary cause with O2 sensor problems farther down the probable cause list. If both the front and rear O2 sensors indicate codes 43 / 45 and 44 / 46 at the same time, then chances are that you have a fuel supply problem not an O2 sensor problem. It is theoretically possible that both O2 sensors failed at the same time; but, that is a less likely occurrence. If you get just one of 43, 44, 45 or 46 then you could have an O2 sensor problem; but, fuel supply such as an injector problem remains a possibility.

 

[Old Guy]

If everything is ok with the CEL light on, I really suspect your O2 sensors. When CEL is on, ECU goes into a default mode than does not take account of the O2 sensors to calculate the amount of fuel to inject and your car runs fine (with bad mpg thought).



I might be wrong, but even with the CEL light on, the problem would remain if it is a pump/resistor/fuel filter problem. Nsxbaby says it is all good with the CEL on.

I just went back and re read your comments. I agree that the ECU will switch to what the 1991 service manual calls a fail safe operation if it detects a sensor failure. The service manual does not specify the fail safe parameters; but, I speculate that error codes 1, 2, 41, and 42 which are all direct O2 sensor problems might be candidates for triggering the fail safe mode for the O 2 sensors. However, these error codes should emerge almost immediately after doing a reset of the ECU. If the error codes that emerge are 45 and 46 I think these are the long term fuel trim exceeding the +/- 20 % limits and I think it is unlikely that the ECU would stop adding the trim correction to the fuel map when the limit is reached (and the CEL lights) - so no switching to a default mode. As to codes 43 and 44, since it is not clear what triggers those I have less of an opinion. If the ECU counts crossings of 14.7 and doesn't see them then it might ignore the sensor and default to the base fuel map for operation.

I agree that if the drivability problem below 4000 RPM disappears once the CEL is triggered, it seems less likely that the problem would be fuel pressure related.

 

[nsxbaby]

I did not check any codes before I pulled the clock fuse because I thought i would start with a fresh slate since the past 20 years of ownership could have several unrelated codes. I did notice in the service manual 02 diagnosis that it said to drive in 4 th gear at x rpms for an amount of time then go to redline and repeat. This is similar to what my experience has been. If I would just drive at a normal pace I would have no problems. If I would make a few redline runs then go to a normal pace at around 3k then I would start getting the bog until the CEL would come on.
This weekend I have been experimenting to try and zero in on what is going on. There are 2 things that I have confirmed.
1. Fuel pump resistor is hot.
2. Disconnecting the o2 sensor is the best mod ever. It runs great without them connected.

 

[asylum]

1. Fuel pump resistor is hot.

Normal.

2. Disconnecting the o2 sensor is the best mod ever. It runs great without them connected.

Change your O2 sensors and enjoy ! They are cheap. For example : https://www.ebay.com/itm/Front-Oxyg...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649

 

[nsxbaby]

update:
I have replaced the o2 sensor and driven appx 300 miles total over 4 different trips with no issues. I think the o2 sensors may have been the problem.

 

[asylum]

Nice to hear that. Enjoy !