Thread: Fuel temp sensor 3.9 V8 Ahhhhhhh

There is an old saying: "if it aint broke don't fix it"

Well I have just learnt that the hard way!!!!

Gromit II is a 97 3.9 V8 that runs quite well but has a very slight off beat sound on tickover (not quite a misfire but not 100% even on all cylinders) Its quite liveable with but the Fuel consumption is not great and it bugs me me so I have been stocking up with bits as and when I can pick them up at a good price, so I had a new Dizzy cap and rotor arm (genuine) and some high quality HT leads and a few sensors like the CTS coolant sensor and the Fuel temp sensor. Well I fitted the Dizzy cap, rotor and leads, two weeks ago. I retimed the engine and all was well for a day in the Brindabella mountains. Gromit was running well, better than before but still not 100% even. Well, the day following I was at some traffic lights and the tickover started to get erratic and it stumbled and stalled. It restarted OK but tickover was getting very irregular. So when I got home I whipped out the Idle bypass stepper motor valve and cleaned it up liberally soaked it with WD 40 and left that to soak in. Well then I got carried away. Of course it could be the MAF thinks I, So I whipped that off and cleaned that with some brake cleaner. Put a new airfilter in while the MAF was off. Then I thought about the sensors that are supposed to be a pile of trouble, the Coolant sensor and the Fuel temp sensor. Easy enough to change while I've got the spanners out. So i changed those too. When I put the Idle bypass valve back inand tried to start it It was very reluctant and turned over a long time before the engine fired. When it did, it would not tickover and was running so rich it was unrea!!l By now I had no time to mess further and had to run it like that for the next couple of days, but it was getting worse. All the time I'm thinking what could it be that I've missed? Well eventually logic took over and I started backtracking and checking all the things I had worked on, but everything was as it should be until I checked the Fuel temp sensor. The resistance was 4.4K ohms yet the old one on the bench in the garage and at approximately the same temp was only reading 3K ohms. Now this device is a thermistor which means it has a negative coefficient of resistance and the hotter it gets the lower its resistance gets so this brand new sensor I had just fitted was telling the ECU that the engine was just above freezing and no matter how hot the engine got it didn't change, so the fuel was being enriched all the time!!!!! I put the old one back in and guess what, it runs like a sewing machine. I still think the original Tickover problem was the bypass valve but I probably cured that in the first five minutes with the WD40 (for the time being). What an idiot!!!
Moral of the story You should only ever change one thing at a time and
If it ain't broke, don't fix it!!!!.........................
PS don't use pattern parts

Cheers

Gromit

"You just gotta be lucky sometimes"!

Yep, always one thing at a time - you've learned your lesson the hard way.