Revs “taking time to drop”

[frenchie TVR]

Hi guys,

Took the Chimaera out this after she stayed in the garage far too long (over 2 weeks since the last use and I was really missing driving the car)…

So the Mrs and I jumped in the car on Saturday afternoon, drove to Goodwood and other places in Sussex and got back on Sunday (stopped at Fernhurst TVR and looked at some quite nice TVRs including some Griffiths were looking at me asking to be tested and bought…. Have not done it but does have an itch that once will need to be addressed… but that is another story…).

Anyway, as always, my Chimaera works faultlessly (starts first time, every time, even if not used for weeks on end, thanks to my “Battery Brain”) and drives superbly, including when in traffic jams in London in baking weather…

But I noticed one thing, when driving on Sunday on country lanes and letting the throttle go, often the revs would not drop quickly, a little like if something was “stuck” on the throttle… they revs would drop, but not as fast as they usually do…

Could someone tell me what it could be…?

Thanks in advance for your help, Frenchie

[Prizam]

frenchie TVR said:

Hi guys,

Took the Chimaera out this after she stayed in the garage far too long (over 2 weeks since the last use and I was really missing driving the car)…

So the Mrs and I jumped in the car on Saturday afternoon, drove to Goodwood and other places in Sussex and got back on Sunday (stopped at Fernhurst TVR and looked at some quite nice TVRs including some Griffiths were looking at me asking to be tested and bought…. Have not done it but does have an itch that once will need to be addressed… but that is another story…).

Anyway, as always, my Chimaera works faultlessly (starts first time, every time, even if not used for weeks on end, thanks to my “Battery Brain”) and drives superbly, including when in traffic jams in London in baking weather…

But I noticed one thing, when driving on Sunday on country lanes and letting the throttle go, often the revs would not drop quickly, a little like if something was “stuck” on the throttle… they revs would drop, but not as fast as they usually do…

Could someone tell me what it could be…?

Thanks in advance for your help, Frenchie

Mine does a similar thing if on the move and wont drop below 1,400rpm unless stationary.

Hows the exhaust by the way?

[frenchie TVR]

Hi Prizam,

My revs are normal when stationary...

My exhaust is sleeved (not completely, I believe but had some work done by one of the 3 previous owners....) and is in good condition.

Cheers, Frenchie

[Phoenixcat]

Prizam said:

Mine does a similar thing if on the move and wont drop below 1,400rpm unless stationary.

Hows the exhaust by the way?

Mine does the same thing, will not go below 1,400rpm until stationary. It has had a new stepper motor recently so it can't be that.

[Dr Mike Oxgreen]

I believe the idle speed is meant to be held slightly higher when the car is moving, but 1400 RPM sounds too high to me.

Mine holds its revs at about 1100 RPM until stationary, then it settles back to about 900.

[Prizam]

I was guessing it was by design, pretty good for when your in traffic.

[Chimaeraman]

Dr Mike Oxgreen said:

I believe the idle speed is meant to be held slightly higher when the car is moving, but 1400 RPM sounds too high to me.

Mine holds its revs at about 1100 RPM until stationary, then it settles back to about 900.

+1

[lestershaw]

Try starting the car and then turning it off when the revs drop a little and then restarting you might find it settles normally then. it's not the answer but it might be a short term cure
Lester

[KateV8]

Sounds like the same symptoms I've had on and off over 12 months and two entirely different engines, my record is an idle speed of 1900 rpm which made the car truly horrible to drive along country lanes, instead of getting engine braking the car was pulling me into tight bends and really felt like it was driving AGAINST me. Much more knowledgable pepes than I will be along shortly to explain this (Blitzracing, you busy?) but I believe there are many different possibilities, air leaks being one and p'raps road speed sensor. In my case the car's having a Canems ecu next week so hopefully should see the back of the problem.

[DonkeyApple]

lestershaw said:

Try starting the car and then turning it off when the revs drop a little and then restarting you might find it settles normally then. it's not the answer but it might be a short term cure
Lester

My Griff would 'race' occasionally. If I was stationary all I did was slip the clutch a little to the biting point as that would obviously bring the revs down but also reset the tickover level.

I just assumed it was a brain fart from the management system as it was too random to be much else.

Never coast in Tivs so never noticed if it did this while moving or just when stationary.

[blitzracing]

A lot of ills can be put down to the TVR speedo "calibration" box under the dash. Its a crap bit of design and prone to electrical noise causing it it to generate a speed signal for the ECU when the car is stationary, so the ECU holds the idle high, as its only supposed to reduce the chance of locking up the wheels by keeping the RPM higher between gears. The TVR box also goes from 0 to 36 mph (fixed) at 3mph so the ECU gets in a twist. I know Steve Heath has spent a lot of time trying to overcome the poor TVR design.You also need the throttle pot to be set correctly so the car knows its in idle mode. As for air leaks- you need to disconnect the stepper motor when the idle is correct and the engine running. This will remove any ECU control from the equation. If the idle is still erratic then its something else. Worth checking the base idle setting as well.

[Quietlybonkers]

...and now the amateur response .....mine was like yours until the stepper motor was discovered to be damaged, and was replaced. Been a perfect gent since then. I know you said it had been replaced.......but is there any way of checking it's working properly?

[TVR Jim]

Have you tried cleaning the stepper motor? I've had this problem on and off and have also found that disconnecting the stepper once warmed up sorts it too.. Not yet got round to changing the stepper as I don't have a spare £100 knocking about!! HTH TVR Jim

[carsy]

Mine would intermitently tick over high for a while after a hot re start. Taking the stepper out and taking it to bits, drowning it in wd40 would cure it for a few days and then we would be back to high tickover but only after a hot start.

Bought a cheap ebay one for £16 and fine ever since.

Although the stepper appears clean the connections inside which you cant get to get corroded and it becomes sticky in operation.

Mine however is a pre cat, Rover gearbox so i dont have the TVR speed sensor box. Like Blitz says if you have this sensor box, it could be the culprit other than the stepper.

[blitzracing]

You can manually control the stepper to see if working with both the Rovergauge software and cable, or a Steve heath ECU mate. If you want to strip it to bits to clean it:


At its simplest the motor can be unscrewed from the plenum housing, and all the carbon and oil deposits removed from the head of the metering cone, and some use of carb' cleaner on the keyway may be enough to get the unit working. Its also worth cleaning out the metering hole inside the plenum. If it still sticks then more drastic measures may be needed. I have now perfected a way of getting them to bits to clean, without damage:

1) Get the car warm

2) Remove the stepper from the plenum and reconnect it.

3) Block the hole the stepper has come from with something air tight, or even the palm of a hand.

4) Hold the stepper so you can catch the central cone and spring when it comes out.

5) Get someone to start the car. Without the stepper in place it will rev at around 2k- 2.5k (This is noisy!!), but the ECU will try to slow it down by powering the stepper motor and pushing cone outwards. It will do this in a series of pulses, a few seconds apart, until the cone and spring drop out. (assuming they are not completely stuck). It may take a bit of manual help to get the cone to fully release. Now stop the engine and let it cool.

6) Clean all the muck off the shaft and cone, and lightly lubricate the screw thread inside the motor.

7) To reassemble, wind the shaft back into the motor so far, but align the slot in the shaft with the plastic keyway as best you can by rotating the shaft in its screw thread

8) With the engine cool, then reconnect the stepper motor. Power cycle the ignition on and off, (don't start) and the ECU will try to pull the shaft back in each time the ignition is turned OFF. This is the slightly tricky bit, as if the keyway and shaft are not quite aligned it wont pull home, so you may need a few attempts.

In a few cases cleaning does not resolve idle issues, and this can be due to corrosion in the metal bearing in the rear of the stepper housing becoming mucked up or corroded, and this requires a new unit.

[macdeb]

blitzracing said:

A lot of ills can be put down to the TVR speedo "calibration" box under the dash. Its a crap bit of design and prone to electrical noise causing it it to generate a speed signal for the ECU when the car is stationary, so the ECU holds the idle high, as its only supposed to reduce the chance of locking up the wheels by keeping the RPM higher between gears. The TVR box also goes from 0 to 36 mph (fixed) at 3mph so the ECU gets in a twist. I know Steve Heath has spent a lot of time trying to overcome the poor TVR design.You also need the throttle pot to be set correctly so the car knows its in idle mode. As for air leaks- you need to disconnect the stepper motor when the idle is correct and the engine running. This will remove any ECU control from the equation. If the idle is still erratic then its something else. Worth checking the base idle setting as well.

This I didn't know, cheers. [no really]

[rev-erend]

Pretty sure it will be the stepper motor playing it's games.

Jules once suggested putting a cap with a 8mm hole over the feed pipe as a solution .. the stepper is still retained but it stops it having so much air to mess things up.

Worth a try ... I just blocked my air feed pipe completely.

[Podie]

rev-erend said:

Pretty sure it will be the stepper motor playing it's games.

A decade of TVR ownership and never once cleaned a stepper motor.

Am I missing out?

Almost convinced it's placebo effect.

[Chimaeraman]

Try changing the coolant temp sensor. Lucas part SNB 802. Costs about £10 and takes 2 mins to do. I had a 'shunting' problem and the change fixed it.

[SSPPGG]

also check the throttle pedal...the outer sleeve on mine once jumped out of its socket down by the pedal, the effect was it held the throttle open for a little bit when you took your foot off, before then returning to throttle closed position