[T3] Temp sensor and MPS problems on new motor

Jessica Chase jesassica at gmail.com
Thu Feb 18 16:28:43 PST 2016


A month or so ago, my husband and I installed a new (stock) motor. After 2 smooth break in sessions and a few short drives, we took the car on a road trip that we’re still on! We installed our AFR gauge for the trip because we wanted to monitor how things were running. 

Using the AFR gauge, in the fall, we were able to diagnose that we were running lean with a new TS2 we installed. Adding 100Ω of resistance to the sensor helped things out and everything was fine until yesterday. On the new motor, we have the same TS2 and MPS (as well as all other FI components) as on our previous motor, but now we're having trouble getting everything to get along.  

At the outset of our trip, with the 100Ω resistor, the engine was lean hunting at warm idle, so we tried a 200Ω ballast and resolved that issue.  However, AFR readings were slightly leaner than optimal, so we wanted to perfect the system. 

I know not everybody on here (or maybe nobody on here) runs an AFR sensor with the FI motor, but at warm idle, there should be an AFR of 13.1 and ours was 14.2. There were no symptoms beyond getting slightly too good gas mileage of 28mpg. But that’s on winter gas in NYS and we barely get that in the summer on highway trips so that’s cause for suspicion.

Today, we decided to investigate. 

First, we got the timing and idle speed in spec.  The idle had been too high and the timing too advanced. Once we dialed it in, the MPS we drove over 300 miles on yesterday would no longer idle.  It would lean hunt at high AFR and then stall. What could that be about?

Second, we tried a different MPS - even though we ditched that one over the summer because it had a vacuum leak and gave us strange readings.  Jim checked it out and installed a new o-ring, but it still gives strange readings…everything from 12.8 (rich) one moment to 18 or 20 (way lean….with hesitation and bucking) with no change in speed or load.  
Third, we tried a NOS unit, but it, too, wouldn’t hold idle for more than a short while without stalling.  

Since TS2 required the 100Ω ballast over the summer to be happy, we tried all three MPS’s with both 100Ω and 200Ω ballasts.  We also tried all three of them with nothing inline.  With the 100Ω ballasts and with nothing inline, they all lean hunted and stalled.  With 200Ω ballasts, two of them lean hunted and stalled.  The third (the one we ditched over the summer for being inconsistent and wonky) idled richly and then continued to give strange readings.  The only way we could continue to drive was to use this third MPS with the 200Ω resistor even though it produces strange results. 

We removed TS2, let it cool off, and checked resistance readings of it and another one we have.  Readings suggest it’s fine: they are within 150 ohms of each other at “room temperature.” We also checked fuel pressure, and it’s fine too. 

It looks like we’ll have to adjust the MPS, but does the situation we’re having sound familiar to anyone and did they find a solution? Our issues with FI just seem to get weirder and weirder…..!!!

Anyway, we have to drive 300 miles tomorrow and we don’t want to have to get towed so we’re not too lean all the way back to NYC!!!





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