Hard starting new Walbro 255 early model 2 to 1 pump conversion

[rcfanatic]

As preventative maintenance, I just had a late model tank installed in my early model 325 along with a Walbro 255 Pump, and the high pressure pump deleted. The additional level sender was installed as well so my late model cluster would read correctly.

The car idles and runs great but I'm having problems with starting. It will crank forever and fire slightly in the process. If I stop and restart immediately it will start after a second of cranking. I was told that this is because the DME stops the ignition after so much cranking, so when I try again it is already primed and fires right up. Although this doesn't explain why it will keep firing slightly during the first attempt.

My mechanic believes that there may not be a check valve in the pump, and this is causing a loss of pressure in the fuel rail after the engine is shut off. Does this make sense for a Walbro 255? Does there need a be a check valve on the fuel return side as well?

Any input appreciated!

[jlevie]

If the pump is working properly and there are no other problems, normal rail pressure will be achieved in just a few turns of the engine even if the rail is empty. And you can check to see if that is happening by tee'ing a pressure gauge into the rail supply line.

The DME doesn't cut spark at any point in extended cranking. You have some other problem, quite possibly intake leaks. Which can be diagnosed by a smoke test.

[dnguyen1963]

Also check your FPR...make sure the diaphram is not ripped.

[rcfanatic]

Sorry I should have said that the car started perfectly prior to the pump being changed. FPR can be ruled out. It idles and runs fine.

If it was an intake leak large enough to hinder starting, wouldn't it idle like crap?

Should the cranking fuel pressure be the same as when running, ~38 psi?

[rcfanatic]

Update. Twice now I've let the pump run four about 4 seconds with the ignition in the on position and the engine does not hesitate to start afterward. I'm guessing I need a check valve

[rcfanatic]

Does anyone have a good source for a check valve? The one used on the E32's is nla.

[jlevie]

Update. Twice now I've let the pump run four about 4 seconds with the ignition in the on position and the engine does not hesitate to start afterward. I'm guessing I need a check valve

The fuel pump only runs when the DME sees timing data from a rotating engine. If the pump runs when just the ignition is on the car isn't in the stock configuration. To know what is going on and thus what needs to be fixed you need to tee a gauge into the rail supply line and connect a test light across the pump power connections.

[rcfanatic]

To know what is going on and thus what needs to be fixed you need to tee a gauge into the rail supply line and connect a test light across the pump power connections.

What should the pressure read while cranking?

[603Racing]

It should be the same pressure as your regulator. And Jim is right, the pump should only have power while cranking or running.

If everything is working correctly you really shouldn't need a check valve. It may take just a bit longer to start, but only a few rotations.

[rcfanatic]

It should be the same pressure as your regulator.

It seems to me that the vacuum during cranking is considerably less than at idle

[rcfanatic]

I've come to the conclusion that the hard starting only happens when the engine is warm. When cold it only has to turn over for a couple of seconds in order to start. Warm it will crank all day and do nothing on first attempt, then key on/start immediately after and it fires right up. Does the DME stop supplying fuel at some point during cranking to prevent a flood? What would cause this behavior?

[603Racing]

I'd refer back to Jim's original post. You may have a vacuum leak. Smoke test.

You could also try a different DME if you have one on hand.

Possibly an electrical issue, as heat can affect resistance, but this doesn't sound like a typical electrical issue.

[rcfanatic]

I'd refer back to Jim's original post. You may have a vacuum leak. Smoke test.

You could also try a different DME if you have one on hand.

Possibly an electrical issue, as heat can affect resistance, but this doesn't sound like a typical electrical issue.

How could it possibly be a vacuum issue? I didn't have this problem prior to the new pump, and the fact that it starts immediately on second attempt doesn't point toward vacuum leaks.

If anything I'm starting to think it could be a bad fuel pump relay that does not pushing enough current when hot, and/or a general lack of current to the pump while the starter is turning and acting as a sink

[dnguyen1963]

I had the same problem a long time ago. 1st crank...nothing, turned key off/on...2nd crank...started right up. I traced the problem to a loose elbow going into the throttle body by smoke test. Carb spray did not reveal the problem. Actually, this elbow did not always have a leak because it could move slightly in and out. Whenever it was in, there was no vacuum leak. JB weld the sucker and you will not have this problem again.

[rcfanatic]

OK, I'll do a smoke test on Monday. Can anyone explain though the mechanism of why the leak causes it not crank *only* the first time? If it's too lean the first attempt, why does the second crank correct the mixture?