TPS at fault?

[c20xe_westfield]

Hi there,

I bought a Westfield with a C20XE Vauxhall engine, Weber 45's and MJLJ ignition last year. It was a semi-finished project, it ran and drove, just not very well. My plan was to finish it off. A bit of background to my problem:

• 1. It ran terribly when I first got it. Fuelling I assumed, as the carbs hadn't really been set up.

• 2. I took it to John Clarkson's rolling road where they set up the Weber's - it ran great, 173bhp. He wasn't able to access the MJLJ with his serial cable, just wouldn't connect, but as "it ain't broke" we didn't try to "fix" it.

• 3. Oulton Park track day. It was misbehaving right from the warm-up lap. John Clarkson's had commented on the ropey ignition leads, so we fiddled with them for a bit and the problem seemed to go away. It ran great all day.

• 4. It's never run right since!

The symptoms - idles great, drives fine at part throttle throughout the rev range BUT heavy throttle around mid-revs causes it to mis-fire. Very heavy throttle causes the engine to cut out, the rev needle drops to zero, then if I coast for a few meters it'll come back to life again, sometimes with a huge bang & flames!

This weekend I fitted new ignition leads to try and fix the problem, then new plugs. No joy. I've got two possible suspects lined up as the source of the problem:

Suspect 1: The crank position sensor - http://i820.photobucket.com/albums/zz12 ... 0425-1.jpg
The bracket isn't the sturdiest, and I've never seen the sensor mounted that way before? But it doesn't wobble at all, and would it have ever worked if the orientation was the problem?

Suspect 2: The throttle position sensor:
http://i820.photobucket.com/albums/zz12 ... 164550.jpg

It doesn't actually turn! I drilled the hole in the back of the bracket to see, and it doesn't rotate. It's too big for the Weber spindle. But then how would it have ever worked OK at Outlon Park & the rolling road?

Unfortunately I've tried 3 laptops & 2 USB>Serial adaptors and haven't been able to connect to the MJLJ yet!

Your wisdom & input is welcome!

Cheers,
Chris

[brentp]

Vibrating sensor bracket is probably the #1 cause of misfire at mid rpm range, as the weaker brackets tend to vibrate around mid-rpm range. This confuses the EDIS module - it loses sync with the trigger wheel, then re-gains sync.

You can run the engine without the megajolt connected - if you still see the problems when revving, then it's likely the above. Make the bracket extra rigid- extra thick metal, triangulated bracing, whatever is necessary.

On the Megajolt, are you able to observe the ignition advance change as you vary RPM, using a timing light?

[c20xe_westfield]

Thanks Brent, after my post I found another describing the same symptoms which helped confirm suspicions and focus my efforts.

I've spent this evening in the dark & freezing cold tinkering by torchlight with various things. The TPS needs a proper mount (Webcon do one in the UK for Weber 45) but it was a simple washer behind the crank sensor was all that was required to fix the problem. It was too far away as it was.

Still more tinkering to do, but it's now at least enjoyable to drive.

Thanks for all your help! Wonderful product by the way!

Cheers,
Chris

[brentp]

awesome! glad you got it working, and thanks for the follow-up!

[c20xe_westfield]

I sorted this all out over the last few weeks, and ran it again on the dyno today...
...the now correctly operating TPS and a proper ignition map gained me +10bhp!

New TPS mounting:



New VR sensor mounting:



Very happy! I'll post the map onto the library too so others can benefit