Cold Start and Possible Fuel Pressure Trouble

[RobotMartian]

I’ve run into a strange cold start issue, and a forum search hasn’t turned up anything. When cold the vehicle refuses to start and initially has a pretty strong fuel odor. After cranking for a bit and letting it sit, and cranking again (roughly 10 minutes) it will thud, thunk, struggle, and finally come to life. After it has warmed up sufficiently, turning it off and back on results in normal starting and operation. I have pulled the cold start valve and checked that it does indeed correctly blow fuel when starting cold. I tried the grey/blue plug swap method and it did get the car up and running. Strangely though, if I totally disconnect the warm up regulator and leave the blue (correct) plug in the CSV (with the engine cooled overnight) the car with pop, make a backfiring sound, and come to life running normally (almost always on the third crank attempt). This seems to be the only way to get the car to start quickly when it isn’t hot. Is my WUR bad?

For full disclosure, I just got finished doing a water pump replacement. The project included replacing all vacuum lines (triple checked that they are routed and seated correctly), spark plugs (correctly gapped), spark plug wires, cap and rotor, cleaning the WUR screen of dirt, and new fuel lines (9 of 13). I also swapped out a spare RPM relay to no effect. The fuel pump is a new combo unit installed 6 months ago. I wanted to get a second opinion before just throwing parts at it.

[David T]

I would start by thoroughly testing the cold start circuit. Make sure the CSV fires (you said you did) but also make sure it can only fire during cranking and that it times out. Recheck the vacuum lines to the CPR and verify the check valve is oriented correctly. Might as well check the vac lines to the thermal switch under the intake manifold too. Next, you might have a bad connection in the ignition circuit and not getting spark when cold. The next time you can't start cold, verify spark. To check the CPR (aka WUR) get a fuel pressure gauge on it and watch it as the motor warms up. Typically when it fails the car starts OK (mostly because of the CSV) but has no power and runs badly as it warms up. Once warmed up it runs good.

[Bitsyncmaster]

Good engine starting is dependent on the mixture being correctly adjusted. Did you check your mixture with a dwell meter after you did all that work?

[RobotMartian]

Thank you for the leads and solution. I verified the vacuum lines, check valve, and thermal switch routing (cheap snake cams are wonderful). Turns out the system was set way to lean (thanks for the suggestion!). I've got the dwell bouncing around 20 on the 8-cylinder scale. To help me understand the K-Jet system a bit better I have a couple questions. Is it possible for the system to be able to compensate when hot for a specific mixture ratio (the car sat happily at 775 RPM when hot), but unable to do so when cold? Further, I noticed that the spray from the injectors was weak when the system was also supplying fuel to the cold start valve. Was that simply a coincidence, or would the total fuel to the system be limited by the mixture screw?

[Bitsyncmaster]

Once the O2 sensor warms up the lambda system adjusts the duty cycle to hold the mixture at 14.7 AFR. So if you have the mixture screw adjusted correctly there is some range that the mixture can be correct. That range of mixture setting on my car is less than a 1/4 turn of the mixture screw.

A cold start and about 2 min. of warming up the lambda system holds 50% duty cycle on the FV until the O2 sensor warms up. So the mixture setting on a cold engine has no range of adjustment.

Also when the wide open throttle switch is on the lambda system holds 60% duty cycle on the FV. So your mixture screw setting has no range of adjustment at WOT.