2nd gen RX7 erratic perfomance problem

[greendot]

Looking for some pointers as to where to look for the cause of this problem.
Trying to establish a baseline prior to making some changes to an ITS car. Its an 86, with 86 computer, injectors and harness but 89 engine and AFM. AF is constant around 14:1 but one dyno pull gives 125 HP and the next gives 165 HP. We think that for some reason the system may be pulling timing back. Other than a problematic CAS (is this likely to be inconsistent?), what sensor(s) would cause the ECU to do this if they were going bad?

[seckerich]

The ECU will pull timing if the temp sensor is seeing too high a reading or is giving false signal. CAS will usually be the cause of a very jerky line on the dyno. Did you wire a 89 AFR into the 86 harness? Ambient air temp readings should be the same but never checked to see if they have the same output ohms per airflow.

[greendot]

Thanks Steve.
We were thinking a temp sensor. There are 3, aren't there?
Yes to the 89 AFR into the 86 harness. It's been this way for years without an apparent problem and have done other cars also without a problem.

[seckerich]

The one you need to check is the green one in the back of the water pump housing. The other is for the dash reading and the sensor on the intake is air temp.

[Weaver7]

Steve at what temp will the ECU start pulling back the timing?

Bill Weaver
ITS RX7
#63

[racingralph]

you can start by setting the timing at 28 degees btdc (no more). the sensor (water temp)is set at 170 degees. that one drove me nuts for a few races , replaced it , car went back to running great !

[seckerich]

Normal temp for timing to retard is over 220 usually, but the sensors start to drift and are a big reason some cars loose power in a race. A fresh one and clean contacts in the connector are a good investment if you run a stock computer.

[Weaver7]

If the sensor removed and will this affect the timing....my intial thoughts are no since it isn't reading anything. Does that sound correct?

[Eagle7]

If the sensor removed and will this affect the timing....my intial thoughts are no since it isn't reading anything. Does that sound correct?

Nope. The sensor is a variable resistor in a circuit that is attempting to measure its resistance - normally by using a resistor dividor to convert it to a voltage. Cold = high resistance; hot = low resistance. Not connected = very high resistance.
I don't know the details of the stock ECU, but from a circuit standpoint, disconnected looks like as cold as it gets. It's possible that the ECU recognizes this situation and chooses some default temperature, but it's certainly not optimal.

[tdw6974]

The one you need to check is the green one in the back of the water pump housing. The other is for the dash reading and the sensor on the intake is air temp.

Steve, Should we replace both sensors? We are experiencing waaaaaay to lean conditions something seems be be shutting down fuel supply. Thanks T weaver

[Weaver7]

I don't believe we are losing fuel as the preasure never dropped of but the higher the RPM the leaner it became showing 16 to 17 on the AFM. I believe it might be the TPS sensor is out of wack but I don't understand why this would cause a lean condition. Getting acceptable numbers with moderate throttle however under WOT it leans out and towards the end of our dyno time began breaking up awful. I really has me confused. The first pull only managed 129HP . Checked for a vacum leak but never seemed to find one....possibly a leak around the primary injectors?

[Weaver7]

Well this weekend went to hell found the vacum leak and fixed that but still WAY down on power....Would lean out around 7K and refuse to pull. Bumped the fuel preasure to 47 and ran a little better. Have replaced injectors with matched set fuel pump is new...temp sensor new....TPS is new (and set properly)...fuel filter replaced compression is good timing set. new plugs and wires...Talked to another S4 driver he said his car was leaning out on the front stretch at Poconos and he bumped his FP to 55 and that seemed to help. What the hell else is there left? ....Any ideas??? Have the 326 box with only the rev linmiter taken out...Any thoughts are appreciated.

[tbtapper]

Well, a reasonably priced option is to purchase one of the tuneable ECU's from Chris Ludwig.

I was chasing all sorts of weird fueling anomalies and actually had the opposit situation in that I ran rich and the motor went all soggy at high RPM.

With a wide band and the programable ECU all is well and can be pretty much dialed in to exactly what AFR I want at what RPM. In a race application pretty much interested in WOT but there are other pieces of the map that are valuable to smooth out also.

I've NEVER been successful with such high fuel pressures as some other folks recommend. I run on the high end of normal (40's) and granted I'm not a Huffmaster but my car does pull strong and clean with all it's got to offer.

Talk to Chris (Ludwig Motorsports) the ECU makes a world of difference.

Travers
ITS 19

[Weaver7]

Thanks after thinking about it this sounds like the most logical choice will get ahold of Chris. The S4 seems to be quite a bear to work with when trying richen the mixture up. I have heard the S5 run pig rich.

[C. Ludwig]

The problem with playing with fuel pressure is that it's a gross adjustment across all load and rpm ranges. It's a huge compromise. If you're already fat at low rpm but need to richen up the high end you end up being way too fat down low. There is a better solution.

Have you changed the fuel filter? The S4 TPS won't effect the WOT operation. It's a narrow range sensor and sees WOT after about 30% throttle anyway. It doesn't know 35% from 100%. The water temp sensor won't effect an RPM range. It would be like a fuel pressure change and would effect the engine at all load and rpm ranges.

[Eagle7]

Any news on this problem? Hope you got it sorted out.

[tdw6974]

We hope it has been corrected! Found that a short(occasional) in the wiring harness was sending the ECU into limp mode!! back to Pocono on July 4 weekend.TW