92 400 Griff, missfire between 2-3k rpm

[TVRJones]

Hi, sorry if this has already been asked (i'm sure it has) but I couldn't find a thread.

After a 50 mile motorway run (which was fine) I started the car after a couple of hours and it's running like a dog.

The problem is that it will cut out when running down to idle i.e. running to a stop at a junction. Also, pick up from idle misfires and keeps missfiring up to about 2.5k rpm - after that it's fine.

I've had the stepper valve out and given it good clean.

Any ideas?

[whitewolf]

sparkies?? I know it seems strange but start with the most common.. spark, fuel and electrics

[TVRJones]

True, I'll check out the basics - thanks

[paul2000]

Hi could be could also be fuel pump relays they are in the nearside footwell they are silver with red stripe down them change them over see if that sorts them out if it works get 2 new ones ,keepone of the old ones that works for a spare hope that help,s paul sorry for the gramma had a few beers

[MrsFlipFlopGriff]

I've had a cut out problems and was told by a well known TVR expert it may be the road sensor behind the dash. Apparently its a 4 inch square black box and a wire may be loose.
FFG

[TVRJones]

Thanks for the advice. I've ordered couple of relays and I'll check out the speed sensor. I'll post back once the works done. Thanks again for the pointers.

[TVRJones]

Hi there, well the new relays are in but the problems still there.

I'm thinking it's not the speed sensor as the speedo is good and accurate which I believe uses the same sensor?

The problem has actually got worse.

I ran the car with the step valve un-plugged - tick over went up to 2500 rpm and settled down over time. The mis-fires were still there though and it would stall every time the revs dropped.

The problem is much more pronounced when the car is up to operating temperature.

From cold it has run well. Though going to start it from cold today and it would not start - a few splutters only.

Should I look back at the stepper valve?

Thanks, Richard

[Colin RedGriff]

Sounds like it is worse when it is warm, could be the ignition amplifier or coil.

[Clippy]

My Chimaera was doing something similar and I solved it by checking the plug extenders - one had 'walked' round and was touching the block.

You'd think that it would have meant that it was mis-firing all the time but no, only at 2,600 - 2,800 rpm when warm, the rest of the time it was fine...

Giles

[Hedgehopper]

As your engine runs better cold and worsens as it becomes hot could it be that the temperature sensor is faulty and is telling the ECU that the engine is cold all the time?

[blitzracing]

Pull a plug out from each side and look at the colour. If its an electrical misfire they will go black on the side that has the problem. If both sides are black its possibly coil or amp, but more likely something like AFM. A fault code reader should pick up something this bad.

[spend]

Clean the engine & battery earths anyway. Battery posts, negative connection under transmission tunnel, engine ground under alt - os chassis (near os engine mount), engine sensor grounds at rear of ns cylinder head.

Electrics make all the engines go around, making these common connections tip top will make most cars work better

[FlipFlopGriff]

TVRJones said:

Hi there, well the new relays are in but the problems still there.

I'm thinking it's not the speed sensor as the speedo is good and accurate which I believe uses the same sensor?

Thanks, Richard

I wasn't talking about the speed sensor by the diff. It is called a road sensor (or so I am told) behind the dash. Haven't actually looked at mine yet but will be doing as I have an intermittent stalling problem too.
Maybe way off mark but i've not seen any opther post relating to it before.
FFG

[blitzracing]

FlipFlopGriff said:

TVRJones said:

Hi there, well the new relays are in but the problems still there.

I'm thinking it's not the speed sensor as the speedo is good and accurate which I believe uses the same sensor?

Thanks, Richard

I wasn't talking about the speed sensor by the diff. It is called a road sensor (or so I am told) behind the dash. Haven't actually looked at mine yet but will be doing as I have an intermittent stalling problem too.
Maybe way off mark but i've not seen any opther post relating to it before.
FFG

Theres a very specific circuit under the dash that looks for pulses from the diff sensor to reach a certain rate as the car starts to move. When it sees this minimum rate, it turns on another circuit that produces a fixed pulse rate thats fed into the ECU whatever speed the car is doing. This is a frig' that prevents the ECU from speed limiting the car as the car never seems to go any faster. There have been a few cases where the circuit continues to pulse the ECU even when the car is stopped as the basic circuit design is poor. If this fails the idle control becomes very erratic (normally too high) as the ECU thinks the car is still moving, so does not control the idle setting via the stepper motor. It wuld be difficult to test without looking at the signal as it enters the ECU, but your fault does not smack of this at all.

[spend]

blitzracing said:

Theres a very specific circuit under the dash that looks for pulses from the diff sensor to reach a certain rate as the car starts to move. When it sees this minimum rate, it turns on another circuit that produces a fixed pulse rate thats fed into the ECU whatever speed the car is doing. This is a frig' that prevents the ECU from speed limiting the car as the car never seems to go any faster. There have been a few cases where the circuit continues to pulse the ECU even when the car is stopped as the basic circuit design is poor. If this fails the idle control becomes very erratic (normally too high) as the ECU thinks the car is still moving, so does not control the idle setting via the stepper motor. It wuld be difficult to test without looking at the signal as it enters the ECU, but your fault does not smack of this at all.

THERE are at least 3 very different implementations of this depending on your diff / gearbox / instrument installation...

You are talking rubbish, the speed limiter is removed on all TVR chips (I can certainly recall MA confirming that), many cars have a direct drive to the ecu from the gearbox (LT77) and drive to the speedo a separate sensor from the diff (GKN). Not all cars have the magic 'splitter' black box;)

FFG's 2 cars are certainly different & I suspect his 'own' car works quite differently from others being a pre-cat, but I can't recall the exact idiosyncrasies of those cars.

[FlipFlopGriff]

It is a precat and probably differnet to opther precats given its history. I was told that it could be this by Dom at Power who said on a number of occasions he'd spent loads of time sorting odd running issues out and it was dodgy wiring to/in this sensor so he now checks this first.
FFG

[spend]

FlipFlopGriff said:

It is a precat and probably different to other precats given its history.

Thats what I was trying to say politely There could be bloomin any perms of a 100 in yours I wouldn't presume to guess

[blitzracing]

spend said:

blitzracing said:

Theres a very specific circuit under the dash that looks for pulses from the diff sensor to reach a certain rate as the car starts to move. When it sees this minimum rate, it turns on another circuit that produces a fixed pulse rate thats fed into the ECU whatever speed the car is doing. This is a frig' that prevents the ECU from speed limiting the car as the car never seems to go any faster. There have been a few cases where the circuit continues to pulse the ECU even when the car is stopped as the basic circuit design is poor. If this fails the idle control becomes very erratic (normally too high) as the ECU thinks the car is still moving, so does not control the idle setting via the stepper motor. It wuld be difficult to test without looking at the signal as it enters the ECU, but your fault does not smack of this at all.

THERE are at least 3 very different implementations of this depending on your diff / gearbox / instrument installation...

You are talking rubbish, the speed limiter is removed on all TVR chips (I can certainly recall MA confirming that), many cars have a direct drive to the ecu from the gearbox (LT77) and drive to the speedo a separate sensor from the diff (GKN). Not all cars have the magic 'splitter' black box;)

FFG's 2 cars are certainly different & I suspect his 'own' car works quite differently from others being a pre-cat, but I can't recall the exact idiosyncrasies of those cars.

Sorry I seem to have missed the bit you have seen when I said said all TVRS are made this way?? I was simply explaining what the box did and how it worked as it had already had the finger of suspicion pointed at it, and blaming it for a mis fire really is not the correct way to go. It was never a statement of "all" TVRS do it this way, and please feel free to explain why TVR should go to the effort to building this box of tricks, other than being not to worry about certain aspects of the maps / speed sensing??? . Dom at TVR power did say to me that they did not use MA for their chipping work, so who knows whats in all the different maps. Just for reference heres the signal trace from a stationary car with a faulty box of tricks.

[spend]

OP's car is a 92 according to his profile, gives a pretty good clue what's in it. You might as well be talking about some kind of Morgan for the relevance...

[TVRJones]

Wow, thanks for all the different angles and advice.

I'm thinking that this might be a good process to follow:

1. The basics, plugs, leads, distributor - after the rough running I'd like to what state the plugs are in anyway. Give the throttle butterfly a good clean. Check earling points.

2. Get one of Steve Heath's error code boxes and see what that says.

3. If fault codes, check relevant sensors first. If OK, check wiring.

4. If no faults, check coil etc. I'm guessing if they are breaking down under heat that I should get some freeze spray and cool them individually so see if problems reduce?

If I track it down, I'll let this thread know. If not, I'll be back with more questions I'm sure :-)

Does this sound like a good first pass? Any ommissions or other thoughts would be welcome.

Thanks

Richard