Starting problems when warm

[Fraggle]

Hello, i have this TR6, its a late car, 125bhp model, it has a lightened and balanced bottem end, stage 2 head and a 150bhp cam, K+N filter and a sports exhaust, including manifold, uprated metering unit from prestige, along with a bosch pump, the injection pressure has been set to 108psi also has electronic/ luminition ignition. When cold we crank up oil pressure, which happens quickly, then start it, it starts fine on all 6 cylinders and runs lovely, it has plenty of power and doesnt miss a beat, untill we turn it off and leave it for a while, like at a car show, when its left for approx 4 or 5 hours when we go to restart it doesnt want to know, it can take up to 20 secs of cranking to get a cylinder to fire then the others catch up, slowly, once running its fine again. All the electrics are in good condition, the injectors are new. Does anyone have any ideas?

[john.r.davies]

Fraggle,

How long does it take to run smoothly after it fires?

Does the time bother/inconvenience you?

 

I have a similar engine, that starts up every time like that from cold.

I can count the cylinders in as they fire up; 3/4-5- - -6!

And I rather like it, as I rouse my dragon from sleep.

Makes it seem somehow less a machine, more an animal.

 

Are your injectors new/reconditioned?

Have you inspected their spray cone?

And that they don't dribble?

If they are the ones with a needle sticking out, then pull on that, until the spray is even, and doesn't dribble.

Mine are fine, but a leaking injector will need to be pumped up before it fires, or a poor cone may not atomise properly until warmed.

JOhn

[JohnC]

First suggestion - search the forum - this chestnut has come up many times before.

 

I'm slightly confused by your comment about building oil pressure and then starting. Is your normal cold start procedure to isolate the fuel pump, crank for oil pressure, and then turn on the fuel pump? If it's not, and you're referring to a one-off, then read on...

 

If this problem happens every time you start after a longish time with the engine off (whether the engine is warm or stone cold), then it does sound like you have airlocked injector pipes. Best way to check this is to remove an injector and see if the cone-shaped spray is present immediately you start cranking. If it doesn't appear in a second or two, then here are a few ideas to get you started:

1. Do you have metal braided injector pipes? Some have found that these conduct heat very effectively from the engine (e.g. via the bracket on rocker cover) to the fuel, causing vapourisation and then an airlocked pipe. Replacement with the standard items would cure this.
   
2. Do the injectors leak after the engine has stopped? Over a few hours this would lead to an airlocked pipe. Replacing the 'O' rings and ensuring the injector cones are free from debris would cure this one
   
3. The non-return valves in the MU could be shot, although this is unlikely given that you have a recently-overhauled unit.
   

Hope this helps - but let us know how you get on.

Cheers,

John

[Richard Crawley]

Fraggle; the others have covered it pretty well & there are loads of archive posts on this topic. I’d go with pressure drop &/or fuel evaporation in the injector lines after hot soak & there can be several causes. As has already bee said, braided injector lines won’t help. Replacing the injector ‘O’ rings usually brings an immediate improvement but they don’t perform at their best for very long & I change mine every year, sometime sooner if I have one that’s causing persistent problems (use only Viton seals). Bleeding the injector lines may also help but don’t expect too much, this is a common 6 trait & almost impossible to permanently eradicate. As long as the motor fires on all 6 after around 5 -10 seconds I wouldn’t be overly concerned; my 6 has always done this & sometimes a persistent injector won’t kick in until I drive off! I find if I start the motor from cold, run it till all are firing & then switch off immediately i.e. before the engine has a chance to warm up, the next start sees it firing on all 6 straight away!

 

How are you priming up the oil pressure? Cranking the engine with the fuel pump running but ignition dissed for extended periods will wash the bores with fuel, dilute the oil & do more harm than good!