Sploogle

Im trying to diagnose a hard start condition when my truck has been up to temp and then cooled off for 1-3 hours. It'll start perfect in the morning, if its been turned off for less than an hour or of it been sitting for more than three hours. It cranks but doesn't fire up, but tapping the gas while its long cranking makes it start right away and once it turns over it runs perfectly.

I've been looking into 4crawlers article and forum posts on re-routing the FPR directly to the manifold to bypass the VCV switch. He says doing so solved a warm start problem. His photos and instructions show two hoses and the switch functioning as on/off. My truck has three hoses on the vcv, one to the FPR, one to the back of the manifold, and one to the very front on the other side of the butterfly valve. Its a 95 with california emissions and the diagram under the hood shows it being routed this way.
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Could it be that the VCV is connecting the FPR to the front of the intake where there is low vacuum on start up when the butterfly valve is closed? Would this explain why opening the butterfly valve with the throttle makes the truck start up? Its EFI so pressing the gas peddle shouldn't do anything to the fuel if its not running yet.

​​​​Anyone have any information on how this should function and how it could be bypassed? The factory service manual linked on this forum doesnt go into much detail on the FPR and only shows 2 hoses.

Thanks.

scope103

The FPR Vacuum switching valve is not "on-off," instead it switches the FPR between regular manifold, and atmospheric (through a little foam filter). Atmospheric pressure is higher than manifold pressure, so when switched to atmospheric it raises the rail pressure, which compensates for vapor lock.

So I think your estimate is correct; instead of switching to atmospheric through the tiny foam filter, yours is switching to the port on the OUTSIDE of the throttle valve. Since that just goes to the air cleaner, that point should always be at atmospheric (or very slightly below) as well. I don't think the function has changed; it's just getting atmospheric air through the air cleaner.

Your problem doesn't really sound like an FPU issue, but it's easy to check. Just run a "test" line directly between the manifold and the FPR, bypassing the FPU VSV. If I'm right about the line to the port on the throttle body just being filtered atmospheric, you wouldn't even need to plug it (but if I'm wrong, you do, so go ahead and plug it!) This MIGHT be a long term solution if it works, but you might have some hard starting when hot (vapor lock).

Sploogle

I tried running it to manifold today and it still had trouble starting. I didnt know about the filter on the VCV but that explains a lot. Might try looking into the temp sesor next.