TPS drifting voltage...

[KnifeySpoony]

I've been dealing with a part throttle/low load hesitation/misfire on my 420R (w/roller barrels) - 

I thought I had it figured out after realizing that my TPS voltage had drifted up. It's supposed to be 4.62V at idle. I found that it was 4.71V and this seemed to be the cause of my hesitation issue. Resetting it to 4.62 quickly allowed the engine rev normally. However I saw that the voltage was hunting a bit just sitting there connected to easimap, so I decided to replace it. The first sensor I put in I set to 4.62, then did a short drive. After the car got warm (about 5-10min) I noticed the idle speed had crept up to 1800rpm (normally ~1100rpm). Sure enough when I got back, the voltage was 4.55 (too low a voltage causes the ECU to think more throttle is applied as the voltage from the TPS goes down as the throttle is opened). So I thought maybe that sensor was bad. I got a new OEM Ford TPS sensor and swapped it in yesterday, set to 4.62 of course. Took the car out again and noticed that it was still doing the part throttle hesitation, although just a tiny bit and barely noticeable. Totally driveable, not the severe misfire it had before, and only over a tiny range of the throttle sweep (ie dead spot between 10-12% throttle, where before it was severely sputtering between 1 and 40%). However the idle speed had gradually crept up to 2000rpm. I got home (with the car still hot) and checked the voltage, which was now 4.51V. Needless to say I am confused. The sensor is fastened tightly - I don't think it's moving. Could this be a temperature issue? The TPS does get quite hot being mounted directly to the roller barrel assembly. I don't think Ford intended this part to get this hot. Could the lower voltage be caused by increased resistance in the wiring with heat? I have jiggled the wiring while connected to easimap and didn't see the voltage change at all.

 

Any thoughts are appreciated.

 

 

[KnifeySpoony]

BTW I reset voltage when I got home with the car hot. Will check in AM with car cool to see if it changes again.

[MV8]

Did you push the throttle body linkage toward the throttle body idle stop while checking the voltage idling? 

Did you check for intake leaks hot and idling?

Did you set the rpm, closed throttle gap and each idle screws per the service instruction provided by Sltous before raising the idle to 1100rpm?

 

 

 

 

[KnifeySpoony]

The barrel spring is very strong as slams hard against the stops naturally. It seems I might indeed have two separate issues - first the drifting voltage of the TPS. The other is that even with the new sensor set to 4.62 (where it seems to stay briefly) I was still getting some mild hesitation. This makes me think about an intake leak or perhaps some imbalance of the throttle bodies. I have not messed with the air bleed screws at all- they are as set from factory. As the barrels wear in, do these need to be rebalanced periodically?

 

I think my first order of business is to sort the TPS drift as that is more a known issue. On the uk forum someone suggested that since the TPS pot is just modulating the 5V voltage coming into it, to test the integrity of the input voltage. Also, it occurred to me that I can test the TPS voltage not just through easimap, but right at the back of the connector. This will help me to isolate the wiring as a potential source of drift (increasing resistance etc). Will report back after some tinkering today.

[KnifeySpoony]

Checked voltage this AM after car cooled. Voltage still at proper setting after adjusting yesterday afternoon hot. Went for shortish drive, came back and voltage stable with car hot. I don't get it. 

 

Line voltage at the plug was 4.92V - which was on 2 out of the 3 pins (not sure I understand that). Not sure if that being under 5V is significant or not.

[MV8]

It is a 5vdc reference voltage as measured against the third pin. It is just a reference and has a tolerance. Even if it is exactly 5.00 vdc at the pin of the ecu, it won't be the same at each sensor due to wire condition, length, and connections. Close enough to provide a reference back to the ecu. The feedback level that you adjust for will richen or lean but that is not a good way across the board under all conditions. It isn't enough to cause an 800 rpm jump at idle from being slightly off specification. A cts has a similar effect on mixture but it is not linear across the ecu input range.

I would not expect oem driveability with such a short runner intake. The service instruction allows you to verify the intake is setup properly.

[KnifeySpoony]

I can say definitely that having the TPS idle voltage setting at say, 4.55 vs 4.62 immediately causes the idle to rise. Given that this car doesn't have a MAF, and that the MAP doesn't really act like a MAP in other applications, I guess the TPS is the only thing telling the ECU how much fuel to inject. The engine and power delivery is very smooth and tractable with the roller barrels actually. The throttle response is very linear but instantaneous.

[sf4018]

The 4.55V vs 4.62V is only ~120RPM delta, it may be a red herring with the TPS being the problem. There are fuel modifiers in the ECU for Inlet Air Temp and Coolant Temp. Would be worth data collection of all the sensors to see if one of them is jumping around when the issue happens.

[KnifeySpoony]

I don't doubt that there could be a secondary issue. Perhaps the TPS voltage is interacting with whatever that is. Rotating the TPS while idling immediately bumps the idle speed though. My ambient air/pressure and coolant values in easimap look good with the car cold in garage - I haven't checked hot.