1993 Massive Hesitation at 2,500 & 5,500 RPMs...
 

All,
I am working on a friend's car trying to work out a few issues. I started working on the car a few months ago and was able to fix a few items, ABS light on, SRS light on, tach/speedo/odometer not working. All of those issues with the exception of the odometer have been fixed. A new issue popped up upon finishing those items, however.

I am getting a massive hesitation/loss of all power at 2,500 and ~5,500 RPMs. It is hard to get it to do it in first gear in my limited time driving it. In second gear, I fairly consistently get a hesitation/loss of power at 2,500 and 5,000 RPMs. In third gear I fairly consistently get the same at 2,500 and 5,500 RPMs and in fourth gear I fairly consistently get the hesitation at 2,500 RPMs. I don't have the cojones to hit 5,500 on a public road in 4th, but I am assuming I would get the same result. I am not sure why the difference in RPMs in second gear (5,000 vs. 5,500), maybe it has to do with the calibration of the tach itself?

It seems to do this both in and out of boost, however, the car doesn't have a boost gauge for me to verify that. It seems to get worse as the engine gets warm over the course of 10-15 minutes, then it happens almost every time I try to run through the RPMs. I have not tried to free rev the engine to see if it stumbles then, I will try that in the next few days.

I was thinking it was a grounding issue. I replaced the negative terminal with a new one that is very snug. I also added a ground strap from the negative terminal to the intake manifold. I double checked all grounds, everything appears to be good.

I was initially thinking a bad knock sensor, but I am not sure. Would that just retard timing, or pull fuel or spark altogether? From what I have read, it retards timing, which would not explain a complete loss of power. I am fairly certain it is losing spark or fuel because it is a sudden, complete loss of power. Another thought was a coil going bad?

Any help/ideas anyone could send my was would be much appreciated!

Are the symptoms aggravated by electrical load i.e. the AC on, headlights/wipers, etc? If so, I would suspect the grounds need to be cleaned. There is a TSB that prescribes cleaning the main engine ground just below the AC compressor, removing the grounding bracket near the driver's shock tower where the battery negative leads to and cleaning that, as well as replacing the ground strap from the engine hoist location on the passenger side to the firewall. I had hesitation/bucking issue at 2000-2500 under light throttle/cruising that was aggravated by electrical load and cleaning those grounds solved it. Otherwise, the TPS is a big one for bucking and hesitation. As long as your voltages are right and there are no dead spots, the TPS should be good.

Matt

Check all grounds. What ecu are you running?

Stock ECU. The car is bone stock from what I have been told, with the exception of a JDM swap some time ago. The CEL is on, but has been on since the swap from what I am told, so I don't think that relates. I will probably try to pull the codes to see exactly why the cell is there.
I will go through and recheck all grounds as well as replace ground from UIM to firewall.

The one time I drove it for an extended period was at night, so lights were obviously on. I will try to find some time during the day to compare and see if lights make it worse.

Yeah, you really need to pull codes. The Check Engine light isn't lit up just for fun. Some errors will put the ECU into limp mode and do all kind of bad things.

Dale

I'd try a different ECU

Possibly switch yours in

Ha! Fritz, I don't know that the ECU in my car would make it run any better!! :)

Dale, The CEL was on well before this issue cropped up, but I agree, need to pull the codes to see if anything else has popped up.

I will update as soon as I get a chance to mess with it some more.

LOL

Forgot you have the LS car :)

See if you guys can find a stock ECU or even better a PFC with a base map

Oh and if it's hesitating massively just when the boost starts and won't go over say 4500 RPM you are in limp mode and it's probably for a good reason.

However PFC doesn't recognize limp mode so beware. In other words OMP may not be working etc...

If the CEL is lit due to the JDM engine swap, then that's a clue something is not right about the swap itself- it will have set an error code related to this. The error code is likely related to the power loss issue and the light.

In simple terms, the CEL and the power loss symptom are very likely related and you need to read the stock ECU for codes.

Also, the CEL could have been lit for one thing, then a while later something else happened and now you have another code. It takes a jumper wire and 1 minute to check codes, no reason not to do it.

RX7 FD & Series 5 Error Codes

Dale

So I checked the CEL tonight, I am getting 11-Intake Air Thermosensor, and 23-Fuel Thermosensor. From what the owner said, the code 23 has been there. I am going to check wiring tonight to see if there is anything obvious going on with it. Could an intermittent issue with the intake air thermosensor cause the symptoms I am getting?

Thanks for the input!

When my IAT sensor was bad my car ran pretty rough. It stumbled alot and had similar issues to what you described in the OP. I replaced the sensor & everything ran much smoother afterwards.

It just seems strange to me that the hesitation is at specific RPMs, apparently regardless of boost/non-boost states.

I would think if it were a bad IAT sensor, it would be more random, unless the ECU is in fact putting it in limp mode at those particular RPMs based on certain criteria...

What if you hold the rev at 3.5 4 rpms does it smoke aswell?

mine does the same thing here and there it still runs and drives but cant keep it in one gear to high or it smokes not much power in 4-5th gear mabee turbos are shot?

So the IAT sensor was not plugged in, thus the CEL. I got it plugged back in, code 11 is no longer there, just the fuel thermosensor code. The hesitation/power loss is still there. Any ideas where I should go from here?

Bigtuna, no smoke here.

You check the tps?

Check the fuel temp sensor..? Its on the secondary rail.

It is a JDM engine, and from what I understand, that is the reason for the code 23. I will do more research on that front.

I will be checking the TPS within the next few days to rule that out.

Thanks for the help on this everyone!

I checked the TPS today. Voltages at closed and WOT were within range. There was a dead spot on the narrow range wire (up high, near 4 volts), but from what I have read that is used for idle purposes only.

I drove the car again today, it does it like clockwork, 2,500 and 5 to 5.5k rpms. Even in 5th gear, it loses all power at 2,500, it is like ignition is pulled altogether. I guess my next steps are to follow the matrix in the FSM. Any other thoughts?

One other observation, the knock sensor, which is on the front housing, has leaking out all of it's 'goo'. It has even leaked down onto the trailing plug. Could the knock sensor be cutting ignition because it thinks something is going on at those particular RPMs?

I had thought I would own another rotary powered FD at some point, but now, I am not sure. :)

Remember, the problems you're having can happen to ANY engine, this is not a "rotary-specific" problem.

I had an FD I worked on a while back that had bad cutting out, it would actually do it free-revving the engine. It was a bad ignition harness. The original harness has a ground wire that goes to one of the hold-down bolts on one of the coil packs, the newer replacement doesn't have that ground wire.

Fuel thermosensor mainly handles switching the vacuum solenoid for the fuel pressure regulator. I don't know how it fails if it's not reading. The fuel thermosensor should be on your fuel rail and the JDM ECU's and harnesses support it, why are you getting that code?

Dale

Yeah, I know. It is just that my supple, LS wiring harness has me spoiled!! :)

This will free rev all day long, it has never cut out on me just revving.

I am not sure about the fuel thermosensor code. The owner told me that code was there due to the JDM swap. I have not done any research to validate/refute that.

I think I need to remove the UIM and inspect the rats nest/injector harness/ignition harness/fuel thermosensor. Just peeking down there I can see at least one open vacuum line, and, based on how stretched the IAT sensor wire is, the harness is not positioned the way it was intended.

I don't know who did the swap, but it leaves a bit to be desired.

IIRC there is supposed to be an open vac line in the rats nest... may have something to do with automatic:
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Does anyone have any more input on this? I still have not dug into the fuel thermosensor issue, trying to leave that as a last resort as I don't really want to remove the UIM if I can avoid it. That code existed before this issue cropped up anyway.

One thing I was thinking about is the fuel pump resistor. Doesn't the fuel pump voltage get bumped up when certain parameters are met? Does anyone know what those parameters are? I am not convinced that is the issue as I still think it is ignition related given the complete, sudden loss of power. I would think if the fuel pump lost power, it would be a more gradual power loss as fuel pressure bleeds off over the course of a few seconds.

Thanks again for any input.