Intermittent limp mode (?), no CEL

[Kid]

Hi,

Not OP here. I'm doing an auto engine/auto trans swap out with a manual engine/manual trans. I've got the swap done, all running and driving but the car has some odd issues which I've never experienced before on anything I've worked on. I'm not experienced on Hondas so I thought one of you knowledgeable people might have come across the problem and be able to help.

The problem is that the car intermittently hits a limiter of sorts at 1400 RPM in any gear except first which seems to go to ~2000 before doing it. When it does this, it loses a lot of power but can keep driving. If you keep your foot into it, it slowly builds revs and power starts to come back as cylinders start to operate again and then by about ~3k all cylinders are back and revs out cleanly. When it's running fine, it has no problem revving out which suggest the engine is mechanically fine.

The car does have the ABS and TCS lights on, which I'm now sure is contributing/causing the problem. The CEL and the charge light flash occasionally when it's in this 'faulted' operation mode. I had a quick look through the troubleshooting guide of the TCS section of the manual and I couldn't see anything about a 1400RPM cut or anything that's close to what the car's doing.

Has anyone experienced something like this before and if so, could they provide some guidance on what could be causing this and what checks to perform, without fixing the ABS and TCS yet (unless that's is the problem for sure)?

Thanks.
-K

 

[Old Guy]

Did you transplant the manual ECU to the car or are you using the auto ECU? There are differences, one being that the auto ECU needs an in-gear indication from the transmission. The auto ECU also has different fuel maps because of the difference in the cams between the auto and manual.

The year of the original engine and the new engine would also be of some assistance because of the changes with the TCS with DBW.

 

[Kid]

Still using the auto ECU but with the map selector pull up/pull down resistor cut. Can I just tell it that it's in drive or will that then cause problems with starting it? Should I put it into neutral instead?
Also, would something like that be intermittent? It sometimes drives fine and sometimes it does this from the get-go.

The car is a '91. Not sure about the manual engine but as far as I understand, the accessories from the motor that was in the car (which is not the original motor but I think it is at least the original TB) got moved over onto the manual engine before I put it in.

 

[Old Guy]

The MT ECU has a clutch switch and neutral position switch. The AT ECU gets a park/neutral indication from the shift lever position indicator. Part of those functions on both ECUs is to put the ECU into closed loop idle control when you are in neutral or park. I also thought I read something in the service manual about the ECU limiting throttle opening if the car was not in gear; but I can't find it so maybe its just my imagination. What have you done with the signals to the ECU that used to come from the shift position indicator?

I have heard that the hardware for the auto ECU can be converted to a MT ECU with the snip of a single resistor. However, my recollection is that the VTEC lobes of the AT versus MT engine is different (hence lower horse power for the AT engine). Different VTEC lobes mean that the volumetric efficiency is different and the fuel maps when in the VTEC operating range need to be different for the two engines. So, you can convert the hardware; but, you also need to revise the fuel map unless you transferred the camshafts from the auto engine into the manual engine. Run manual engine VTEC cams with an auto ECU with the original fuel map and I think you are going to be running seriously lean in VTEC mode.

Your comment about the MAP pull up resistor confuses me. Normally the MAP sensor gets a fixed 5v supply from the ECU and then feeds a variable voltage back to the ECU that is proportional to manifold pressure. Pull up or pull down resistors are typically used when sensing the status of a contact and would not be in the MAP circuit. Perhaps that is a pull up resistor for the parking / neutral indication from the auto transmission selector switch?

If the problem is related to the manual versus auto ECU, I would not expect it to be intermittent.

Have you used the service check connector to retrieve the error codes in the ABS, TCS and the ECU. If the CEL is lighting up on occasion, you may have stored codes. Getting the codes may give you some indication as to what is going on.

 

[Kid]

The MT ECU has a clutch switch and neutral position switch. The AT ECU gets a park/neutral indication from the shift lever position indicator. Part of those functions on both ECUs is to put the ECU into closed loop idle control when you are in neutral or park. I also thought I read something in the service manual about the ECU limiting throttle opening if the car was not in gear; but I can't find it so maybe its just my imagination. What have you done with the signals to the ECU that used to come from the shift position indicator?

I haven't done anything with these. Cars I've converted to manual from auto in the past haven't needed thing done to the ECU so I haven't done anything with this one either, although I do understand it is an NSX and it's probably smarter than a 4A-GE or an SR20 ECU.

I have heard that the hardware for the auto ECU can be converted to a MT ECU with the snip of a single resistor. However, my recollection is that the VTEC lobes of the AT versus MT engine is different (hence lower horse power for the AT engine). Different VTEC lobes mean that the volumetric efficiency is different and the fuel maps when in the VTEC operating range need to be different for the two engines. So, you can convert the hardware; but, you also need to revise the fuel map unless you transferred the camshafts from the auto engine into the manual engine. Run manual engine VTEC cams with an auto ECU with the original fuel map and I think you are going to be running seriously lean in VTEC mode.

Yes, this is the thing I was talking about in your point below. As far as I understand, cutting that resistor changes the maps within the ECU to the 'manual' maps and should change to the manual engine fuel and timing maps which should take into account the different VE of the two engines. I don't know if my assumption is correct or not though. I should probably buy a spare harness for my wideband so I can pull the controller and sensor out of my car and swap it onto another car for cases like this.
We did buy a manual ECU so I'll swap that in. We had it in he car at one point but it didn't alleviate the problem we were experiencing at the time so we put the auto ECU back in.

Your comment about the MAP pull up resistor confuses me. Normally the MAP sensor gets a fixed 5v supply from the ECU and then feeds a variable voltage back to the ECU that is proportional to manifold pressure. Pull up or pull down resistors are typically used when sensing the status of a contact and would not be in the MAP circuit. Perhaps that is a pull up resistor for the parking / neutral indication from the auto transmission selector switch?

Sorry about the confusion, see above.

If the problem is related to the manual versus auto ECU, I would not expect it to be intermittent.

Yeah. This is what's bugging me. I can't reliably recreate the problem. I'm thinking of disconnecting the ABS and TCS (and the fail-safe relay) and seeing if one causes it to go immediately and reliably into the 'limp mode'.

Have you used the service check connector to retrieve the error codes in the ABS, TCS and the ECU. If the CEL is lighting up on occasion, you may have stored codes. Getting the codes may give you some indication as to what is going on.

I was wrong on this one and the owner corrected me - it's not the CEL but the cat over-temp light which flashes up at the same time as the charge light.
No, I haven't pulled codes lately - I did when the second engine was in the car (and auto) and was running terribly. I'll pull the codes from the various control modules next time I'm at the car but the car owner is convinced the ECU is fine since the CEL has never stayed on like the ABS and TCS. The CEL did come on once from memory but it cleared itself soon after.

 

[Larry Bastanza]

Get the codes, sounds like it may be a wheel speed sensor......

 

[Old Guy]

Enabling separate fuel maps by snipping a link is possible. I am limited to the description of the ECU in the service manual and have never bothered to investigate the ECU code. sr5guy has posted the characterization for the pre OBDII ECU code, so you might be able to confirm the presence of fuel maps for both the auto and manual by looking at the characterization file. Ultimately, if the AFRs in VTEC mode look safe, then that is all that probably matters. The transmission selector connection may just indicate to the ECU to go into closed loop idle control mode. Since idling is not the immediate issue, whether or not that connection is correct may not be relevant to your current problem.

The OEM NSX does not have a cat over temperature light and the OEM ECU does not have any provision for measuring the temperature of the catalyst. The engine MIL / CEL light is just below the charge indicator light. Again, I would use the service check connector to see if there are any stored codes in the ECU.

If the voltage is steady in the car around 13.8 volts when it is running the charge indicator light should not be flashing. If you check the section of the service manual dealing with the charging system, you will find that problems in the ABS controller (internal shorts) can cause the charge indicator light to light up. Follow the charging system test procedure to confirm that the charge indicator light flashing is coming from the ABS unit. Problems in the ABS frequently migrate to the TCS because the TCS gets its wheel speed signals from the ABS. If the TCS and ABS lights are lit up continuously, that indicates a fault in the systems or that the TCS is shut off). Normally, if the ABS and TCS are in permanent fault mode (the ABS and TCS lights are constantly lit up), the systems become inactive so they should not cause your problem. However, it the TCS light is flashing then that indicates that the TCS is active and could be causing your problems. It is possible that you might have some intermittent problem causing the TCS to activate. Have you tried switching the TCS off to see if that remedies the problem? Once you access the error codes for the TCS and ABS you may get a better indication of what is going on.

An additional test would be to disconnect both the ABS and TCS systems to see if that addresses the issue. If the problem remains with both systems disconnected , you are back to looking at something else.