Rough start when warm

[402Z]

Hello everyone,

Ever since I bought the car it has been doing this for some reason. I recently replaced the injectors and the car runs GREAT. Except for one thing, if I drive it, get it all warmed up, and it sits for about 45 mins to an hour the car has a pretty rough startup for a bit and then it seems to go away after about 30-45 seconds. It stumbles pretty bad and then it eventually smooths out and there are no other issues while driving it. Only does it when the car is warm and it has sat for about an hour after driving? Cold starts and perfect and warm starts are fine besides when it sits for an hour then it does that issue. What could the problem be?

[XfireZ51]

Hello everyone,

Ever since I bought the car it has been doing this for some reason. I recently replaced the injectors and the car runs GREAT. Except for one thing, if I drive it, get it all warmed up, and it sits for about 45 mins to an hour the car has a pretty rough startup for a bit and then it seems to go away after about 30-45 seconds. It stumbles pretty bad and then it eventually smooths out and there are no other issues while driving it. Only does it when the car is warm and it has sat for about an hour after driving? Cold starts and perfect and warm starts are fine besides when it sits for an hour then it does that issue. What could the problem be?

What was warm start up like before u replaced injectors? And what were the old injectors and what did u replace them with?

[Young1]

I had bad stumbling and missing after a 30 minute heat soak and discovered two corroded coil and plug wire terminals. What I did to troubleshoot was to hook my old timing light to each plug wire while idling. It was obvious what was missing by the light flashes. There were long carbon tracks on the coils to the mounting screw. I cleaned it all up and applied two new crimp on terminals to the plug wires. Left over old fashioned stuff from my Ford FE days. Everything ohms ok. No dollars spent. Saved the gaskets. Will reassemble on Saturday. Bryan you have a coil or plug wire problem aggravated by heat.

[Paul Workman]

I would be interested in results of an injector fuel pressure leak-down test.

Put a pressure gauge on the rail and turn the ignition switch to RUN position - long enough for the fuel pumps to charge the rail. (Somewhere in the 45?). The pressure should hold to within one psi for several minutes (10 minutes~). If it drops off - especially down to zero - you may have a leaking injector - which would give you the rough start-up issue. (But for the record, defective check valves in the fuel pumps can also cause a drop in pressure*. But, that wouldn't cause rough running on startup after sitting idle for a few minutes.)

*both of these results - leaking injector & leaking fuel pump check valve - are from personal experiences.

[gtcollins@maxnet.co.nz]

Could also be leaking fuel pressure regulator diaphragm

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