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MG MGF Technical - Lumpy Tickover & Fast road Cams

I have had fitted some fast road piper cams (hydrolic type with 268 duration) plus veriner pullies. The engine now runs lumpy on tick over. The garage tried to get the engine to run smooth but could not. They played with some sensor on the Throttle Body by moving it back and forward as well as the throttle position. They said that it happends on some MGF's and Lotus Elise due to too much air being drawn into the engine and hence a week mixture.

Another problem is that if I Go fast and use high revs when I take my foot of the accelarator and the revs drop down the engine sometimes stalls. I have noticed that the revs drop to about 600 then increase very quickly to 1300 then settle to 900. It is when it drops to 600 it sometimes stalls.

P.S I have an SP exhaust, K&N 751 air filter.

Please help.
James

Hi James,
I've got the same cams without any problems. The mems is a very complex system taking information from sensors all over the engine. The mems on our F's as you probably know is not programmable but it does learn. How long have you had the cams running? My set did cause the same 'almost stall' thing, but it did fix itself after about a week. Yes, too much air causes a weak mix which lowers the revs but the rapid pick up afterwards is caused by the mems compensating. I am currently reading a 70 page document which i gleaned from Dave Andrews' site all about engine management.
Oh and by the way, the Icon unit does work well with these cams (yesterday i refitted the icon after a week without it).
In answer to your question - All engines are different and you might benefit from a new mems unit mapped to suit your setup.

Nick
Nick

>>All engines are different and you might benefit from a new mems unit mapped to suit your setup.<<

Not sure where you'll find one of those (i.e. you can only buy the MEMS with the standard map) - or perhaps you mean an aftermarket ECU to replace the MEMS?

Back to James' problem, a 268 deg. cam isn't wild enough to justify a lumpy idle, especially with the standard heavy flywheel. The MEMS does have a reset procedure which forces it to forget all it's learnt and start again - I can't remember what the procedure is, something like "face north, hold the accelerator fully open, switch on the ignition, pat your head 3 times and chant the magic password for 15 seconds". Perhaps someone else knows the details...

Mike

Mike Bees

I remember Carl from Sweden said a few times that some parameters of the MEMS could be reprogrammed thanks to "advanced toolbox ?" of Testbook.
Don't know if those parameters include fuelling ...

A question for Mark the Tech ?

Fabrice
Fabrice

I had a problem with identical symptoms on a standard VVC - diagnosed (and fixed) as a faulty inlet manifold sensor (the wiring was dodgy). Perhaps when the cams were fitted the sensor was dislodged? (not earthing properly).

N837 OGF
Hugh

I seem to recall that to 'hard reset' the MEMS you had to disconnect the battery for half an hour to discharge the built in memory.

Works with PCs too... ;o)

TBQH, I suspect that Hugh is probably spot on with the sensor malfunction theory- and is certainly well worth getting checked out.

Rob
Rob Bell

You mention Vernier pulleys. Has each cam been set to the correct setting?

For road engines it is so much simpler to have standard pulleys as there is only scope to fit one way!!

The only other factor that may be ultimately considering is that as these 268 degree units are reprofiled, rather than machined from new blanks, there may be an issue of the grind on these specific cam(s).

Rog
Roger Parker

Thanks for all your comments.

In answer to some of the questions.

Nick the cams have been on the car for about a week.

Rog. The cam are from blanks. There was only one verier pulley used and it was set between 3 and 5 degress BTDC, I think. I only quickly picked it up whilst the fitter was trying to do the car.

Hugh. If what you are talking about I think is slightly adjustable by moving it back and forwards. Is it the sensor connected to the inlet manifold next to the throttle buttlerfly?

As at today the car still runs lumpy. I may try to reset the MEMS.

I have also been informed that maybe a pressure boost valve may help by increasing the fuel to the engine on tickover.
James

Have to admit not looking in the engine bay for a while now (no problems!) so I've forgotten. Seems unlikely though that a pressure sensor would be movable by hand, as it could easily vibrate to a place where it may read a 'loca'l pressure (pressure profiles can be really complex in high-velocity situations, and highly transient).

Sorry if this is no help. I should think that if there is an intermittant problem with a particular sensor it should show up on the testbook as random blips.

N837 OGF
Hugh

This thread was discussed between 10/04/2000 and 12/04/2000

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