The series 3 has started stumbling to the point of almost dying on a hot start. Catches right away but the stumbles hard with a low, lumpy idle. A bit of throttle for 20-30 seconds seems to help get it back to its usually smooth (for a series 3) idle.

Any thoughts or suspects?

Thanks in advance,
Rhett

 

Fuel check valve malfunction where it is not allowing the fuel rail to retain pressure after the car is shut off. With the pressure diminishing, fuel vapor is formed in the rail / lines. Vapor lock. This is one cause.

 

An '84 SIII should have the fuel temperature switch in the fuel rail to alleviate the fuel vapor lock possibility....if it's still working, of course . At xxx-degrees fuel temp (150ºF?) the switch dumps the vacuum signal to the pressure regulator. This increases fuel pressure ...and pushes air out and fuel in.

A test would be to simply pull the vacuum hose off the FPR...this replicates what the switch is supposed to do.... and see if the situation improves.

The fuel pump will fully pressurize an empty fuel rail in about 2-3 seconds so that shouldn't account for 30 seconds of poor running, it seems.

Cheers
DD

 

How hot is 'hot'? Twenty minutes of driving? Or 3 hours roaring down the interstate? Both the ignition module and the coil can give trouble if they absorb engine heat long enough.....although you wouldn't typically expect either one to just straighten itself out after thirty seconds.

Are you getting any black smoke form the exhaust when this happens, or have the feeling that the engine is flooding a bit? If so.....

XJ6 Series - EFI Rich Mixture

Among other things you might have some drippy injectors over-fueling the cylinders. A cold engine would like the extra fuel; a hot engine would not.


Cheers
DD

 

Hi Rhett,

Doug is on the money as per usual.

I have been experiencing a similar problem.

In my experience last week my 10 month old Ignition amplifier gave up on a 39 Deg Celsius day. The ignition just cut out. 10 mins later, good to go for 10-15 mins and then just cut out again.

The similarity with your situation is that as long as I kept revs over 1100 rpm the engine wouldn't stall. Not much fun in peak hour traffic.

I previously suspected a cooked coil as even last week it was too hot to literally touch, but it soldiered on ( I swapped it out to be sure and I am not ballasted).

The only other thing I am trying to isolate is whether a worn (read weak spring) distributor advance mechanism is adding to the woes?

Reasoning for looking at Dissy is that from cold start I struggle to maintain idle without stalling. Good when warmed up, but starts to idle really roughly when hot from sitting in traffic. Not unlike your situation.

As long as you are using the air con, vapour lock shouldn't be on the radar as the fuel line runs through the cooling system to help remain cool.

Hope this helps.

Cheers,

Nigel

 

Anywhere from half an hour to 2 or 3 hours. The idle does get noticeably less smooth after a few hours running hard or in traffic but never to the point I think the car might die. It only does the hard stumble at (warm/hot) start.

Yesterday, I imagined I smelled a whiff of fuel as I came off the freeway but it was faint and well gone by the time we parked outside our lunch destination. When we got back to the car less than an hour later, the stumble occurred. I stopped at the bank a few minutes later so my wife could pop some checks into the ATM and the car started normally.

I don't see any smoke. I've tried turning the key on and waiting 10 seconds or so to let the fuel pump prime and I've tried just turning the key without waiting. The only thing that seems to affect is how fast the engine catches but does not affect the idle once it does.

If it was over fueling, would putting down the throttle make it run smoother? I'd imagine more fuel would just kill it, no?

Now that I'm thinking about it, I had very nearly this set of symptoms last year, but replacing the fuel pump seemed to fix it.

I do have the cap mod installed. SARC can confirm this but I believe the ignition coil is less than 2 years old.

 

The 'could be' list is long. You may just have to pick a place to take the first stab and keep diggin' until you hit paydirt.

But, one more idea....

Be ready to pull a spark plug and check the spark the next time the problem occurs. If you get a wimpy orange-yellow spark, as opposed to bright blue/white, then you'll know you have an ignition problem.

For that matter you could rig up a fuel pressure gauge and leave in the engine day....in a safe location....and see what the fuel pressure is when the engine is acting up.



I was thinking that overfueling was loading up the cylinders...and applying throttle and increasing RPM helps the engine clear its throat, so to speak




Just in case you didn't know, there is no 'prime' capability in the pump circuit and the fuel pump should not operate by simply turning the key 'on'....unless someone has made some modifications

Cheers
DD

 

Ah, must have been something I misread. Thanks.