Throttle hesitation (sometimes extreme!)

[itm]

I've got a '94 Griff 500, and have had a problem with throttle "hesitation" for a long time. In the last couple of months it's become extreme - i.e. total loss of power while changing gears, then a sudden explosion into life after a couple of seconds. Pretty alarming, and not a little dangerous.
It's been in to Mole Valley TVR this week. They replaced the throttle potentiometer, but 3 minutes after driving it away I got the same problem.
Anyone else recognise these symptoms??

Ian

[GreenV8S]

quote:I've got a '94 Griff 500, and have had a problem with throttle "hesitation" for a long time. In the last couple of months it's become extreme - i.e. total loss of power while changing gears, then a sudden explosion into life after a couple of seconds. Pretty alarming, and not a little dangerous.
It's been in to Mole Valley TVR this week. They replaced the throttle potentiometer, but 3 minutes after driving it away I got the same problem.
Anyone else recognise these symptoms??

Ian

I would have suspected the throttle pot too, it can cause exactly those symptoms. Does the problem happen every time you change gear, or just occasionally?

Just guessing wildly, it might be worth looking at the vacuum controlled bits i.e. the vac advance, and the fuel pressure regulator. Problems here could show up when you open the throttle. I would start with the vac advance, since that's relatively easy to get at and test. Locate the vac advance tube from the throttle assembly to the distributor, disconnect it from the throttle assembly, block off the port it was collected to on the throttle assembly, leave the free end of the pipe open to the air. Also check that the vac advance mechanism is securely attached to the distributor (they do work loose sometimes). The car will be fine driving like this, but you'll notice the economy goes down on long cruises and will improve when you reconnect the vac advance. If this cures the problem, it is probably a faulty vac advance one-way valve (if you have one) or (less likely) vac advance unit. If it's the one way valve, I'd ditch it completely and connect the throttle assembly port directly to the vac advance unit, that valve is more trouble than its worth.

The other thing to test is the fuel pressure regulator, this really needs specialist equipment though.

If it isn't any of these things, you could potentially be looking at an ignition problem causing a weak spark but I would have thought this would show up as a more general misfire not just when you change gear.

By the way, subtract a couple of brownie points from the people who guessed at the cause, swapped a component and sent you away without checking it had actually cured the problem. I trust they won't be charging you for that.

Hope this helps,
Peter Humphries (and a green V8S)

Oh, PS, if you run out of ideas, check the ECU earth is OK. When this went on mine I got all sorts of wierd behaviour.

[itm]

Many thanks for the advice. It's an intermittent problem, and is not just occurring on gearchange. On one occasion I got jerkiness while cruising in 3rd at 40-50mph. I've also had problems with stalling when the engine's hot.
Do you think this lot could also be consistent with the vac advance theory?

Ian

[apache]

I had this exact problem and it took a long time to find the cause. If it only happens during wet conditions it'll be the same as mine, the bonnet had worn a hole in the induction pipe, rain would run down the bonnet into the pipe and go onto the hotwire sensor causing much distress to all and sundry

[GreenV8S]

quote:
Many thanks for the advice. It's an intermittent problem, and is not just occurring on gearchange. On one occasion I got jerkiness while cruising in 3rd at 40-50mph. I've also had problems with stalling when the engine's hot.
Do you think this lot could also be consistent with the vac advance theory?

Ian

Um no, if it's happening while you're cruising that sounds like a different type of problem. A dodgy throttle pot would fit the symptoms really well, but you already tried that.

The intermittant and severe nature of the problem makes me suspect an electrical or wiring fault. It could be just about anywhere so this is a wild stab in the dark, but from those symptoms I'd suspect a failing fuel pump relay, especially if it tends to happen say fifteen or twenty minutes into a journey. If it is this, it's likely to get worse and will eventually leave you stranded. (Been there, done that, not recommended.)

Although you can swap in a standard 50p changeover relay to get you home, it has the wrong pin configuration and will flatten the battery if you leave it in. The proper relay is a special dual-pole one that is quite hard to source, your best bet would be your local TVR specialist or LandRover dealer. Quite expensive too.

While you're looking around the fuse box it's worth pulling out and re-seat all the relays to make sure nothing is loose, gently pull all the wires while the engine is running to make sure there aren't any hidden breaks round there. This would be a good time to tape all the relays into their sockets if this hasn't already been done, one less thing to go wrong.

Hope this helps sort the problem, but let me know how you get on?

Cheers,
Peter Humphries (and a green V8S)

[nobbie]

My griff was really jerky around 30-40mph (I had to keep dipping the clutch to control it!!). I got it from Castle TVR. They took it back and traced the fault to the vacuum advance unit on the distributer. They fitted a new unit and it's now sorted and running like a dream - Thanks Ian @ Castle!

[gjm]

quote:Oh, PS, if you run out of ideas, check the ECU earth is OK. When this went on mine I got all sorts of wierd behaviour.

i have similar symptoms sometimes. where is the ecu earth located on a 4.3 griff peter?

[GreenV8S]

quote:
i have similar symptoms sometimes. where is the ecu earth located on a 4.3 griff peter?

Sorry, don't know. But early 4.3s have a reputation for being a bit funny about throttle pots, and I gather they used a chip that was too keen to generate lambda sensor fault codes. If yours is giving trouble, I'd consult an expert like Mark Adams, if you haven't already. I think both these problems are well known and fixable.

Cheers,
Peter Humphries (and a green V8S)

[shpub]

The earth is under the central console. Other intermittent causes have included coil, ignition module, temp sensor, lambda sensor, fuel pump (relay, immobiliser), duf timing, faulty ECU... the list is endless.

The problem is in these cases if the simple stuff don't fix it, get the car on a rolling road where they can see exactly what is going on.

Steve