No power at low revs

[chimsouthafrica]

I had my 4.0 chim serviced by the only service station in my city with experience on TVRs.
It now misfires, jerks, and has no power below 2500rpm, once it hits 2500rpm it takes off and performs normally.
They have had the car for two months now running diagnostics on the ecu, and have replaced coil, plugs, ht leads, rings on one cylinder (they claimed lack of pressure on the cylinder) and still can't find the cause of the problem.
Any suggestions would be appreciated.

[sotonS2]

Might be worth searching the Chim forum for 'stepper motor' which can display these symptoms when dirty / defective etc.

[GreenV8S]

Basic fault finding would be the first step, and I expect they've done this. But they should start by checking whether the air/fuel ratio is within limits and whether the spark is clean and at the right time. From your note I guess it started when they serviced the car so it is likely to be something they changed and not a more fundamental problem.

So not much help.

One concrete suggestion they might not have come across yet: the ecu takes a low voltage signal from the distributor in order to work out what revs the engine is doing. This signal is normal carried by a single black wire that runs along the top of the near side rocker cover, under the throttle body. It's vulnerable to interference from the near side HT leads, so you need to make sure they're kept separate. Interference will cause the ECU to work out the revs wrongly, which might cause this sort of problem. But apart from this sort of guesswork, really the only approach is to methodically work out which system is going wrong and why. It could just be a duff earth, loose connector etc but a methodical approach will track it down.

[dlewis]

.......and what did they touch during the service?

[chimtvr]

ask if they adjusted the ignition timing the bottom crank pulley has a shock absorbing fibre bit seperating the inner from the outer so over time the timing mark ends up sliding to the wrong place. If you want to try it yourself mark the position of the distributor then move it slightly one way or another see if things improve if not put it back to your marks used to see this problem a lot on range rovers and discoverys if it sorts out the problem buy a new bottom pulley then have it set up with a strobe
Good luck

[shpub]

If it is that , be careful as the pulley is balanced to the crank and simply putting a new one on could cause problems. Worth looking to see how different the two are and modifying as needed.

The fibre bit is rubber damper that damps down any vibration and they can split but it is rare. when this happens the timing marks will move and go out. Draw a white line across the two halves of the pulley and run the engine. If the line seperates that is your problem.

Steve
www.tvrbooks.co.uk

[chimtvr]

Just a thought but if it has split it will be out of balance anyway so surely a new pulley with timing marks in the right place will be better than one that changes itself???

[shpub]

chimtvr said:

Just a thought but if it has split it will be out of balance anyway so surely a new pulley with timing marks in the right place will be better than one that changes itself???

TVR balance the crank using the pully/damper. If you fit a new one on that is very different from the original, it puts the whole engine balance out. It all depends on how much metal was added/removed from the pulley to acheive the balance.

Yes it will be an improvement as the timing marks will be right but if it puts the engine balance out then it can cause a lot more expensive problems - like worn bearings or even broken cranks. Definitely worth checking before fitting.