TPS Voltage Dropping Under WOT & Knock

[HyperV6]

I raced my car last Friday and had some problems. The TPS voltage was dropping under full throttle. I'm pretty confident I wasn't lifting on the gas. I've got over 400 passes and 10+ years of racing my car and the TPS has always been 4.76-4.77 volts at full throttle on all the passes. I haven't touched the TPS since I set it 10+ years ago. I also remove my floor mat prior to racing to prevent interferance. I was using TurboLink for all the data below.

race #1: started at 4.74 volts - went down to 4.52 volts at the end of qtr
race #2: started at 4.74 volts - went down to 4.68 volts at the end of qtr
race #3: started at 4.70 volts - went down to 4.62 volts at the end of qtr

Any ideas why the TPS voltage would drop? The sensor going bad or bad ecm?

I also had some huge spark retard on the 1st pass. On race #1 the second frame of the pass I got 40 degrees or spark retard. I noticed the rpm was 3950 at 18 mph. On the third frame the rpm was also 3950 but at 37 mph with 6.8 degrees of spark retard. The car seemed like it was stuck in 1st gear and not accellerating. The transmission is original and doesn't shift perfect anymore. The car has acted this way before on the street but no scan tool was hooked up at the time. What would cause the hanging of rpm at 3950 from 18 to 37 mph? Or could this problem be related to the TPS issue or the transmission going out?

My car is an 87 T that has the stk turbo, injectors, downpipe, and intercooler. I was running Red Armstongs 93 chip for stk injectors and pump gas with 14 psi boost. On the 2nd pass I had no spark retard at all. The 3rd pass I had just a few degrees of spark retard in two or three frames.

Any suggestions would be greatly appreciated. I have TurboLink files of the three passes.

Thank you, Craig

 

[Ormand]

What was the alternator doing when the TPS dropped? May be a drop in overall voltage? Low voltage gives low fuel pressure, which gives knock and then other bad stuff. What were the O2 readings?

 

[HyperV6]

The O2 readings were all fine. I think the lowest O2 on all three passes was 780 mvolt range. Almost all the other readings were above 800 mvolts. I checked the alternator and it's output was fine. I do not use any type of volt booster.

What happens when a TPS goes bad? I've never had one go bad.

Craig

 

[1970gsx]

I would not worry at all about a 0.08-0.22 drop in voltage. You are still way above the recommended 4.0 tps needed for PE mode. It's probably due to you being pushed back in your seat pulling your foot ever so slightly off the pedal. My car will do the same. Just my .02....

 

[Lee_Burough]

Just a guess....False Knock?

I would not worry at all about a 0.08-0.22 drop in voltage. You are still way above the recommended 4.0 tps needed for PE mode. It's probably due to you being pushed back in your seat pulling your foot ever so slightly off the pedal. My car will do the same. Just my .02....

Gregg is correct about this, I think the PE mode will work anywhere above 3.8 volts. I have to think that the TPS voltage shown by Tlink is accurate, and shows the TPS working as designed.


There are, I suppose, several things that could impact your driveability in similar fashion....The thing that I wondered about (thinking of the number of passes at WOT and possibility of false knock) is whether or not your motor mounts are still good....Do you use an engine tie down to keep the engine from torquing and hitting the down pipe?

I'd recommend a modern chip with the Turbo Link's 'Fast update' feature. You'll get four times as much tuning and performance data from each cycle of the PCM with the faster update (there is a setting in Tlink for the faster update chip programming).

Just something to consider.

lee

 

[ijames]

The O2 readings were all fine. I think the lowest O2 on all three passes was 780 mvolt range. Almost all the other readings were above 800 mvolts. I checked the alternator and it's output was fine. I do not use any type of volt booster.

What happens when a TPS goes bad? I've never had one go bad.

Craig

But what was the battery voltage reading in the tlink data log while the tps voltage was low? Did it droop, or stay steady?

Usually when a tps goes bad it gets bad spots where the output falls to zero, and most often these are right at or just off idle where it spends most of its time. Or it can just drop to zero all the time, like if you get it too wet washing the motor.

 

[HyperV6]

The battery voltage on my 3 passes were all between 12.9 - 13.1 volts. So, I think the alternator is working correctly. Yes, I use a torque strap. I've installed the torque strap back in the early 1990's. I've looked at the engine mounts and transmission mount back in the spring and they looked ok. I guess the TPS voltage dropping slowly isn't really a big deal. Just hasn't happend since I've raced the car from 1991 till now. I'll try to verify nothing was hitting or rubbing to get this knock.

Could the knock be caused by the transmission? Via TurboLink my car was stuck at the same RPM 3950 for two full frames in 1st gear. That is when the 40 degrees of knock occured. The next two passes the transmission shifted a lot better and I no knock on the 2nd pass and only a few degrees of knock on the 3rd pass.

Thanks for any info, Craig