Hot start problem I've tried everything ...seriously

[zimvsdib]

I have a very regular (like clockwork) hot start problem.

Following all the usual steps suggested in the past threads i have done everything listed below:
Installed inline fuel check valve
replace ppr o rings
replaced fuel accumulator
checked for any fuel leaks on lines
checked control pressure (50 psi) Rest pressure maintains very well
adjusted/tuned dwell mixture to bounce between 55 & 40 (I'm in the zone but is a little tempremental)
Deactivated Cold start when hot (ruling out thermo time switch/ flooding)

I'm just getting frustrated.
After the car is hot i cannot start it. Even tripping the cold start injector i can't get it going (i can smell the fuel when i do this so i know its spraying).

I feel like I'm left with 2 possibilities left to rule out.
1. its leaky injectors (including cold start) that may be allowing the vapor block in the injectors.
2. its related to spark, maybe the heat weakens spark by way of the coil or resistor.

My question is, how do i move forward testing?

I had an idea for a test. Would this determine if it is vapor in the lines:
Get the engine hot, then remove the injectors(put them in jars), then quickly trip the fuel pump via rpm relay. If it IS vapor in the lines, no fuel should spray from the injectors. ???
Any help would be appreciated as I'm tired ...so tired.

[Bitsyncmaster]

Injectors will not spray just running the the RPM relay. You would need to press the air plate down for them to spray.

How long do you hold rest pressure? If you hold it for 4 hours it's probably not a rest pressure problem. That test will help you troubleshoot the problem. You can test rest pressure hold time via taping the air plate when timing if you don't have a pressure gauge attached.

[Michael]

adjusted/tuned dwell mixture to bounce between 55 & 40 (I'm in the zone but is a little temperamental).

This caught my eye.
Mixture adjustments being off just a little can cause a massive hot start issue. Car may idle, run and sound great but fail to start hot.
Also I think your mixture may be a little off. Everything I have read says to try and nail it down to swing center over the 40 degree mark(35-45) which would be 20 on your 8cyl scale. Be sure to plug your mixture screw hole and then watch your meter to see if you have set it right.

Once you are happy take it out for a drive and recheck it. Thinknof the mixture screw as a stopwatch face. One or 2 seconds off and you got a hot start issue...it's THAT sensitive.

When you say it's tempermental you could have a underlying issue such as a clogged cat which will cause mixture adjustments to stray.

[David T]

The next time you have a hard hot restart, do the plug swap. If that doesn't get you going you don't have a rest pressure problem. Check for spark. Most likely causes for no spark are:
bad coil
bad impulse coil
Rusty connections on the ballast resistor
Bad cap or bad ignition wires

[powerline84]

So the hot start problem on my car had dmc midwest scratching at their heads to . Ended up being the vacuum triple switch under Imtake was broke and distributor vacuum was not working properly .

[NckT]

In addition to the above very good suggestions, I've diagnosed only fixed only 2 cars that had a bad vacuum delay valve that with the thermo vac switch in the cold/ vacuum supply to the Control Pressure Regulator, with the naff valve caused the fuel control pressure to be very low ie over fuelled because this condition caused the diaphragm in the lower part of the CPR to be pulled down. This should only occur with a pressure differential when the throttle is opened suddenly under load.

This is different to the fuel screen in the CPR being blocked, or its useless 'sintered bronze bush' filter that bosch use as part of the CPR refurbishment.

I highly recommend getting a fuel pressure gauge to do the tests.

Good luck

[zimvsdib]

Thanks for the tips everybody.

Regarding your questions:

I have an inline pressure gauge hooked up to the WUR to see control & rest pressure.
After an hour and a half it went from 50 to 25 psi. Rest pressure seems fine.
(I have a nice fuel k-jet tester in the mail, not here yet)

I adjusted Dwell again. I'll post a video of my dwell just so you can see how temperamental it is. No dice.


A couple things to note.

Regarding the metering plate as Bitsyncmaster mentions.

If the metering plate does not open while cranking, than the piston does not move thus no fuel will reach the injectors.

Only vacuum pulls the plate open which operates the piston movement allowing fuel flow.

I tested both of my cars this morning(dead cold zero fuel pressure) tapping on the metering plate.
The car with no issues taps the plate down like a seesaw with zero resistance. However the hot start car gives me resistance on the plate. i can press it down but it requires more energy (its far from an effortless tap). I think this maybe my culprit.

I 'll post the videos in separate posts below

My thinking is that the piston has a less than perfectly smooth operation. So it needs more force than the vacuum can provide to drop the plate. Maybe its slight fuel varnish build up on the piston just enough to so it doesn’t have a very loose/smooth piston operation.

I don’t know exactly why this isn’t an issue when i start the car cold (its starts first try ever time) but i think there are a few things happening while its hot that work against it.

As Powerline84 mentions. Loss of vacuum is another culprit. I have all new vac lines but i could check my Thermal Control Valve (triple switch under intake) and my Solenoid valve (small unit sitting behind the mixture unit) are working. As this may also weaken my vacuum pull.

OR it could be electrical like David mentions. I do have a new coil, ballast resistor, & wires

[zimvsdib]

Delorean plate WITH Hot start problem

[zimvsdib]

[Ron]

If I'm understanding it correctly, both vids are with the engines cold. If so, I'd say the one with a hot start problem had better rest pressure than the other (Not that the other one has bad RP, depending on how long it has cooled...)