V6 hot start

[williefixit]

Sorry if this subject is covered elsewhere, but I've failed to find the exact issue.

I've had a 1999 V6 petrol Bongo for 3 years and until recently cold and hot starting have been excellent - always first time, without throttle. However, recently hot starting has deteriorated and takes a few seconds and starts best with some throttle. Cold starting remains always first time and without any throttle. Air and fuel filters were changed last year and I’ve only done 6000km since with a total of 120,000km. A Bongo-friendly garage suggested renewing the spark plugs which I’ve done (Bosch Super 4 FR78X). Whilst I think the engine may be more responsive, there’s been no improvement in the hot start problem. I wonder if it’s related to the fuel or possibly the exhaust system?

If anyone has had similar problems, please share your experiences and hopefully solutions.

[cheffy34]

Sounds like a weak spark to me leads, coil etc deteriate when hot producing weak spark, lots of checking i'm affraid back to basics

[apole]

Hi could be spark or fuel related maybe.

check the distributor cap, ensure all leads are a tight fit and inside check the condition of the contacts, rotor arm and the carbon finger. These can wear down and cause deterioration in the spark quality.

Years ago I have a VW golf that was a pig to start when hot, it was something to do with the fuel evaporating in the carb and having to be pumped through. Probably a red herring here as it's a fuel injected car with a pressurised system.

Andy

[williefixit]

Thanks to "cheffy34" and "apole".

I made myself an adjustable spark gap and connected it in series with one of the plug leads to see if there was any deterioration of the spark when the engine was hot. I found that the spark would jump about 11mm when cold and about 10mm when warmed up, so there was no serious reduction in spark energy as the engine warmed.

In carrying out these tests, I can provide further information on the problem. The hot start problem is only the initial start after the engine has stopped say for 20minutes. Once it starts, it’s OK.

I’m thinking “heat soak” and “vapour lock”, affecting fuel delivery, but where to check next????

[williefixit]

I eventually discovered that under high vacuum conditions in the inlet manifold, fuel was leaking though the fuel pressure regulator into the inlet manifold via the small rubber vacuum tube.

Whilst I didn’t know the fuel pressure regulator was the cause of the hot start problem, it was wasting fuel and unlikely to be regulating the fuel pressure correctly so it had to be changed.

The only way I could get a replacement was to order a new one via Mazda Newport who were very helpful and ordered one from Japan. It cost £216, took a few weeks to get here and I had to unbolt and lift the rear end of the inlet manifold to fit it, but it cured the hot start problem, and should save fuel, so I’m happy.