Car won't start when hot - just turns-over.

Car won't start when hot - just turns-over.

Author
Discussion

HeWhoDaresRoy

Original Poster:

354 posts

152 months

Tuesday 4th September
quotequote all
It's a 1993 4.3 and I've changed the coil, ignition amp and ECU temp sensor, but to no avail.

The starter motor spins freely, but the car won't start unless I put my foot flat on the accelerator.

I don't suppose anyone can think of anything else to check? I'm out of ideas!

ianwayne

1,910 posts

204 months

Tuesday 4th September
quotequote all
There's the old favourite, the stepper motor. If it's seized in the withdrawn position, there'll be too much air when it's cold.

QBee

16,129 posts

80 months

Tuesday 4th September
quotequote all
Not sure about early cars.

In my 99 Chim it would be called the hot start problem, and it is solved by changing the incorrectly wired, and pretty well knackered as a result, Meta immobiliser. Mine did it the day i bought it, but has never done it since Carl Baker swapped the system for the latest Meta and wired it properly.
I found with mine that you could set your watch by it. Stop for a newspaper, get back in, won't start. Exactly 15 minutes later to the second, it starts as if you had imagined the whole thing. Also makes you look silly if you had already called the RAC. That was how long the immobiliser relay took to cool down enough to work, I think.

Also, mine has always starts from cold pretty well straight away.....BUT......if I stall it within a few seconds by being too gentle on the loud pedal, then it can be a long old job to restart it, and flooring the throttle pedal can be the only way to clear the flooding. Just mentioning that for other readers.

ChimpOnGas

8,266 posts

115 months

Tuesday 4th September
quotequote all
The OP describes only being able to start his hot engine by depressing the throttle pedal fully, this is a classic sign of a too much fuel being present, as others have pointed out opening the throttle fully is the traditional method for starting a flooded engine.

You now need to work out why you have too much fuel present, and basically it will be down to one or more of these three issues.

1. A weak or nonexistent spark on one or more cylinder (Failing HT leads, spark plug extenders, spark plugs ect ect)

2. Insufficient air (the stepper motor not opening)

3. Over fueling (ECU issue, sensor issue, or a grounded injector ie fully open)

My money would be on issue one or two, or potentially both, but don't discount three either. So lots to check, don't guess be systematic to prove conclusively where the fault resides rather than blindly throwing new parts at the car in the hope you'll stumble on a cure.

Good luck with it and don't forget to let us know what you find thumbup



NB: Over fueling on hot start may be a faulty fuel temp sender, the ECU will detect when the fuel exceeds a certain temperature threshold and apply a correction percentage to the cranking fuel settings. Basically the injectors will be held open a little longer to compensate for fuel vaporisation in the fuel rail caused by heat soak, if the sensor is sending a falsely high temp to the ECU it may well cause over fueling on hot start.

The above is just a pointer to stimulate further investigation, so don't assume it is your issue and don't just replace the sensor in the blind hope it will solve your problem. Use Rover Gauge that will give you a fuel temp reading and then test the sensor resistance to prove/disprove it is/isn't at fault. If the car is also difficult to start when cold or is easy to flood following a cold start your problem is likely ignition related, IE a weak spark (see issue one in my list of three likely issues) which TBH is far more common than a faulty fuel temp sensor.



HeWhoDaresRoy

Original Poster:

354 posts

152 months

Wednesday 5th September
quotequote all
Great stuff, thanks.

Stepper motor is new, so hopefully not that.

I've got Rovergauge cable and a multi-meter, so will see how far that takes me!
Advertisement

ChimpOnGas

8,266 posts

115 months

Wednesday 5th September
quotequote all
Cold and hot starts are two elements that are hard to get right, the Lucas 14CUX cold start strategy does seem to involve lots and lots of fuel and quite a bit of air too but just touch the throttle and an engine stall from flooding is common, hot starts can be equally problematic for different reasons.

Your hot start issue is nothing to do with the immobiliser because you can start the car with your foot flat on the throttle pedal so that does point towards too much fuel being present, well too much fuel or not enough spark to burn it completely.

When you start testing with your meter it's also worth checking the voltage at the coil, do this under cranking conditions too as the stater motor puts a huge demand on the whole electrical system which will invariably drag your readings down, this could easily make for a weak spark under cranking conditions. Poor earths are also a common problem on these cars as are failed plug extenders and cooked HT leads, actually there are so many things it could be by far your best bet is to check everything systematically.

You may even find the too much fuel theory is a red herring, indeed you may find there's too little fuel present, to check this it's well worth removing a spark plug from each bank when you're having the hot start issue to see if it's wet or not. If the plugs come out dry check your fuel pressure when the engine is running but again especially during hot cranking when the electrical system is being severely tested. A poor supply to the fuel pump is another common issue on Chimaeras, and just like the supply to the coil this issue is compounded when the car gets hot as resistance in any circuit will increase with heat, add in the huge current draw under cranking and it's worse still.

A marginal voltage and or amp supply to the fuel pump may not give you a problem until that is the pump has been run for a while and so has become hot, high pressure fuel pumps draw a surprising amount of current so even though they are cooled by fuel flowing through them they can still become hot to the touch. Feel your tank too, the hot fuel situation is common especially when the fuel level gets low.

When the pump gets hot it's less efficient so the marginal TVR wiring can mean you reach a tipping point where the already weak supply becomes a situation where the pump isn't working within it's design parameters, especially when cranking, low fuel pressure in the rail can be the result and may well be insufficient for an effective hot start. This situation is compounded by the fuel itself being hot too, petrol these days has a very low boiling point so vaporises readily in the hot fuel rail causing pockets of gas to form, your saturated petrol injectors are designed to work with a liquid fuel so any pockets of gas will give you hot starting troubles for sure.

Like I say, lots to check so good luck with it.

HeWhoDaresRoy

Original Poster:

354 posts

152 months

Friday 7th September
quotequote all
OK, I got the car nice and hot (about 93c on the roverguage display) and sure enough, it wouldn't start.

The fuel temp said 35c, so not sure if that's warm or not. I also tested the output of the air mass meter as per this:

https://www.pistonheads.com/gassing/topic.asp?h=0&...

and it seemed ok.

No plug extenders on the car - just some sock things.

May I ask how to test voltage at the coil? I just had a bash and the multi-meter read 3ish volts on cranking, but as the car has cooled down a bit, it started right up! I was expecting 12ish volts, so guess I did it wrong....

Belle427

3,297 posts

169 months

Friday 7th September
quotequote all
HeWhoDaresRoy said:
OK, I got the car nice and hot (about 93c on the roverguage display) and sure enough, it wouldn't start.

The fuel temp said 35c, so not sure if that's warm or not. I also tested the output of the air mass meter as per this:

https://www.pistonheads.com/gassing/topic.asp?h=0&...

and it seemed ok.

No plug extenders on the car - just some sock things.

May I ask how to test voltage at the coil? I just had a bash and the multi-meter read 3ish volts on cranking, but as the car has cooled down a bit, it started right up! I was expecting 12ish volts, so guess I did it wrong....
You should have 12 volts present on the coil positive terminal with ignition and cranking too.
Make sure you earth one probe of your tester to the engine metalwork or a good known negative and not the coil negative terminal.

HeWhoDaresRoy

Original Poster:

354 posts

152 months

Thursday 6th December
quotequote all
Late reply, but....I checked voltage at coil and that seemed fine.

Found that the car starts when hot, but only when you turn the ignition on 4-5 times to prime the fuel pump multiple times. Does that sound like a fuel pressure issue? If so, I can't understand why it would only manifest itself when hot?

Classic Chim

8,528 posts

85 months

Thursday 6th December
quotequote all
Probably a silly question but did you try the coil voltage when the coil was hot. Try the same test when it’s not starting to be sure or at least narrow it down a bit.
Coils are known to be ok when cold but can fail when hot. A weaker spark at this point might be enough to cause the poor starting.
You may have a fuel pump issue, I’d also test the fuel rail pressure when the car fails to start.

You do have a healthy amount of fuel in the tank? Low fuel could be causing crud to block pipe or restrict flow.
Just thinking aloud scratchchin

ChimpOnGas

8,266 posts

115 months

Thursday 6th December
quotequote all
HeWhoDaresRoy said:
Late reply, but....I checked voltage at coil and that seemed fine.

Found that the car starts when hot, but only when you turn the ignition on 4-5 times to prime the fuel pump multiple times. Does that sound like a fuel pressure issue? If so, I can't understand why it would only manifest itself when hot?
Interesting, that's too little fuel then, when you turn the key on the ECU runs the fuel pump for a few seconds to prime the fuel rail, this is the noise you hear, but there's actually more going on. When you turn the key on the ECU also opens the injectors for a pre-determined number on milliseconds to wet the inlet manifold.

The wetting aids starting and you will not hear it happening as you do with the buzz from the fuel pump, this opening of the injectors pre-start is known as the 'Priming Pulse' and you should only need one of them for every start, the fact you can solve your hot start issues by cycling the key multiple times means one priming pulse is not enough, so the starting issue is clearly a lack of fuel.

This indicates an issue with your injectors, a faulty fuel temp sender, a faulty ECU coolant temp sender or as you quite rightly say is a symptom of insufficient fuel pressure at the rail. Use a fuel pressure gauge attached the Schraeder valve on the rail, the pressure here should be 36-38 PSI (2.5-2.6 Bar), any lower and it could indicate a leaky fuel pressure regulator diaphragm of more likely on a TVR Chimaera a weak current supply to the pump.

Earths and generally all the wiring on Chimaeras is poor and the fuel pump supply is one circuit that seems to suffer from voltage drops more than most, start by checking the fuel pressure to confirm the theory, if the pressure is low check for leaks at the regulator by removing the small vacuum hose and check for petrol. If it's dry and doesn't stink of fuel use your test meter to check the voltage directly at your battery and then compare it with what the fuel pump is getting, it's not uncommon to see 12.8v at the battery but only 11.5v at the pump.

This 1.3v drop has been stolen by Mavis who made a terrible job of wiring up TVRs in the mid 90's, this gets no better over the years and the earthing points corrode as does the wiring which only increases resistance and adds to the problem, the good news is with some thicker cable and a decent earth return you can correct the poor work Mavis inflicted our cars wink

If all checks out fine on the pressure and pump supply voltage fronts suspect a faulty sensor, the fuel temp sensor is especially connected to when the ECU runs or doesn't run the priming pulse feature or at least how many milliseconds the injectors are opened for. On hot start modern fuels that vaporise easily will readily turn to gas so you need to hold the injectors open for quire some time to clear this column of gas before a supply of liquid fuel becomes available.

The fuel temp sender in the fuel rail tells the ECU how long it needs to open the injectors for during the priming pulse so the rail is cleared of gas, so if the fuel temp sender is faulty (also quite common) this could very easily be your problem wink

Dave thumbup



HeWhoDaresRoy

Original Poster:

354 posts

152 months

Awesome, many thanks for taking the time to post that-up.

Car is now with local classics garage, as I don't have skills/equipment for testing fuel pressure, so will pass this info on to them.

Hopefully, we'll get it sorted soon.