968 - Reluctance to rev - what to check?

[jcelee]

Hi,

I use my 968 as a daily for commuting and it will run beautifully at sub 3000 revs for 95% of the time - very smooth.
When fully warmed up and when the opportunity arises and I try to take it up above 5000 it becomes rather reluctant to rev, it certainly doesn't feel comfortable taking it anywhere near the rev limiter. I believe the car has a Promax 7100 chip in it but have not yet opened the ECU to check. I also know it has a K&N filter in a standard airbox.

I have been advised on the 968 UK forum to make sure that the airbox and K&N are nice and clean and without touching the wire or getting the electrics wet to soak the MAF sensor in rubbing alcohol (that will clean without leaving any trace). Also to look for any obvious dirt around the throttle body.

Does this all sound sensible? Is there anything else I should look at or check? The belts/chains/tensioners etc have all been replaced in the last 12 months / 5000 miles at a well regarded indy and checked after 1000 miles.

Thanks

[Wozy68]

jcelee said:

Hi,

I use my 968 as a daily for commuting and it will run beautifully at sub 3000 revs for 95% of the time - very smooth.
When fully warmed up and when the opportunity arises and I try to take it up above 5000 it becomes rather reluctant to rev, it certainly doesn't feel comfortable taking it anywhere near the rev limiter. I believe the car has a Promax 7100 chip in it but have not yet opened the ECU to check. I also know it has a K&N filter in a standard airbox.

I have been advised on the 968 UK forum to make sure that the airbox and K&N are nice and clean and without touching the wire or getting the electrics wet to soak the MAF sensor in rubbing alcohol (that will clean without leaving any trace). Also to look for any obvious dirt around the throttle body.

Does this all sound sensible? Is there anything else I should look at or check? The belts/chains/tensioners etc have all been replaced in the last 12 months / 5000 miles at a well regarded indy and checked after 1000 miles.

Thanks

No mechanic, but I'm pretty sure that I read, a knackered CAT can cause the engine not to rev or run correctly. ie a chunk has come away and is blocking the exhaust outlet.

[jcelee]

Thanks, could be a good call, anything blocking the exhaust would have limited impact at low revs but cause problems as the engine breathes more heavily (I know this from when I lost a stud on the manifold of my Caterham and the gasket partially obscured the outlet).

I'll look for any obvious exhaust issues - any way of identifying a broken Cat?

[Trev450]

jcelee said:

I'll look for any obvious exhaust issues - any way of identifying a broken Cat?

You could try putting you hand near the tailpipe and have someone rev it to feel how much pressure there is, or alternatively take it to an MOT station and ask them to do an emissions test.

[fergus]

I'd go for MAF/TPS being dirty, and possibly arcing over the various internal contacts.

Simple to test: unplug the sensor, and put an ohmeter over pins one and three (assuming, p1=ground, p2=+5v p3=signal, although you may have to experiment), then move the throttle butterfly and see if the resistance moves.

[Sandy59]

Might be worth also checking the fuel supply, there's a seperate pump and filter in the line which could cause pressure/flow issues ??

[ClassicCarMad]

fergus said:

I'd go for MAF/TPS being dirty, and possibly arcing over the various internal contacts.

Simple to test: unplug the sensor, and put an ohmeter over pins one and three (assuming, p1=ground, p2=+5v p3=signal, although you may have to experiment), then move the throttle butterfly and see if the resistance moves.

Spot on i'd certainly be heading in this direction.

[dry664]

Jcelee, have you seen my recent thread on 968uk? Ive had some problems but at the other end of the rev range. Might give you a few ideas.

Check Clarks Manual for more info on cleaning the MAF:
http://www.clarks-garage.com/shop-manual/elect-22....

[fastgerman]

If it's an issue while accelerating then could be fuel pump, filter - basically anything to do with fuel/air starvation.

[Lowtimer]

As people say above... but alternative possibilities include camshaft wear, or the Variocam system being up the spout.

[jcelee]

Success! I bought some Isopropyl Alcohol from ebay and thought I'd give the air filter and MAF a thorough clean before looking at anything else. I poured some of the rubbing alcohol through it and used a paper towel soaked in it swiveled through one of the holes to clean it some more (making sure nothing made contact with the wire). It didn't look hugely dirty but certainly some grime came away. I also used the alcohol to thoroughly clean the connections.

Having put it all back together I warmed the engine and tried revving it with the bonnet open. Straight away it seemed to be revving much cleaner. I took it on a road test to confirm it was the same under load and sure enough back to rude health.


Many many thanks guys!

[stratfordshark]

Good to hear you've sorted it.

The K&N filters have a reputation for oiling up the crucial MAF hotwire, or at least unless you are careful not to excessively oil them. I would stick to original!