Ok, this is about to drive me batso.

So i have a new to me 93, it has a nice street ported motor running the stock twins, FMIC, upgraded radiator, PFC, ect all the basics.

Well the motor runs great (PFC is not tuned so not beating on it yet, but other then in boost) then all of the sudden it will start running like junk.

I have traced the issue to funky MAP sensor reading. It will read normal vacume when it is runnign good then it will suddenly start reading .3 bar of boost and obiously run pig rich.

There is no rhyem or reason to when it works and when it doesn't. I drove it around all day today, not a problem, then i was just about home and was revving it a little just listening and seeing how it was running and it starts runnign real bad again, look and sure enough it shows boost on the commander again.

The voltage on the MAP sensor changes as well when this happens.

I tried 2 other ECu's and another known working MAP sensor. Did the same thing with all of them, so it is NOT in the MAP or ECU. Which leaves the wiring.

Being my first FD, i have no clue about the wiring on this car other then there is a dang lot of it. So what should i check? any ideas?

 

Sounds like a wiring issue, probably caused by an abraded harness.

cf. https://www.rx7club.com/showthread.p...sensor+harness

 

That is my thinking, the question is how to find the problem? I have already checked all the wiring i know of, anyone got a new harness? might just be easier to replace the harness then trying ot track this down.

Any common places to check for shorts?

 

The link mentioned the passenger wheelwell.

 

Yep, just got finished checking it and nothing i could see. Plastic appears to be in tact and looks fine

 

Are you seeing the same voltage at the sensor end of the wire as at the corresponding pin on the ECU? You are clearly getting some sort of continuity, but the wire could be intermittently shorting if the harness is damaged. You should be able to trace the wire from the sensor to the wheelwell and then inside the cabin to the ECU to look for damage. It is not as complex as you might think.

A new harness will be a pain to install. I am not sure what the going rate is from them in the classifieds but they are probably not too cheap.

 

Didn't really want ot crack open the harness, on the outside it looks perfect, i know the harness was replaced last year somtime when the motor was swapped, the PO mentioned the harness in particular.

I traced the wire best i could and there was nothing external that i could see. Could it be a ground somplace? i checked the few i could find.

I HATE wiring issues like this. I know it is somthing simple, what it is is the problem.


Far as voltage goes, yes the voltages are the same on the ECU and MAP side except the ground wire has some voltage without the map conected. Which has me confused.

 

A higher voltage on the line means either you have MORE resistance on the wire (voltage = resistance x current) or you have 2 bare wires touching resulting in voltage being where it shouldn't.

I would look for any splices, corrosion on a splice can cause problems like that.

Also, what is the ohm reading when you put one lead on the CPU side, and one lead on the map sensor side of the harness?

The problem you're going to have is this seems to be intermittant if I'm reading your post correctly. I'm not sure how that would happen if you're just sitting there at idle.

 

Yes, it is intermittant which is why it is such a pain. It will work perfect then it is like a switch is flipped and the MAP reads boost while at idle, or pretty much anytime. It is a short somplace, no clue where.

The signal wire appears to be the same voltage at both the sensor and ECU, the ground wire has voltage on it at least when it is messing up i noticed. Which is why my first thought is bad ground. Where are the grounds for the map sensor?

 

You shouldn't have a voltage on the ground wire, as ground is your "reference" voltage.

Test to see if any of the map sensor wires are shorted together by checking resistance to each other at the plug, that'll let you jiggle it around too to test for an intermittant short (like a bad set of headphones). I bet you've got a bad/cracked/corroded plug at one end or where the harnesses join.

 

The FD has a pretty noisy common ground so a (small) ground voltage is not unusual.

I just noticed looking at the wiring diagram that the MAP sensor is on the front harness. You do not want to replace that if you can avoid it (it is $2k+ new from Ray and is a serious PITA to install). This also means that the wire from the MAP sensor is not very accessible and routes all the way around driver wheelwell, behind the front bumper, and back to the ECU in the passenger footwell through the passenger wheelwell. I am not sure that you have any alternative to tracing the harness along this route to look for damage. I would first pull both wheelwell liners and look for damage.

 

I have done some of that but didn't think of the rest, i will try that tomorrow if it stops raining. Thanks.

Thanks, that actually gives me a rather good idea. I remeber seeing some wiring near the front that looked a bit rough, i had not paid much attention to it till now, i will check it over tomorrow.

 

Well thanks alot to whoever it was that figured out the wiring is not in the engine harness but in the harness that goes all around the car.

Started looking around and found that int eh DRIVERS side wheel well it had indeed hit a few wires and frayed them. Fixed them up and it works like new again so far!

Now if i could just get the Blitz IC piping to stay on i could see hw it is supposed to run. under vacume it runs much better though!