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Caterpillar C5000 engine will try to fire up once when start cranking and then not for two or three revolutions, then fire up one more time but not stay running. It had a E27 error code that was already fixed. We have changed the spark plugs, ignition coils and hen changed the crank and cam position sensors as well as the small wiring harness that goes to them.

We ran the diagnostics and checked all pressure sensors. They all seemed to be good. After i swapped ECU and other components from another good running forklift we thought it needed a complete engine harness. That was then replaced and the problem with the truck is still there. It has no codes anymore but when you try to start it fires once and stops, keeps cranking three more times then it will fire once then stop. It tries to fire up at a regular steady rate but will not at all between the three revolutions interval thing. Any ideas would be great.

You may have a fuel pressure problem. Check the pressure next. Make sure the filter is not plugged up. Check the throttle is not full of carbon. Check the diaphragm is not torn, have seen that a few times.

Fuel pressure out of the main LP hose going to the throttle body should read about 5 psi when cranking,

Under your mixer there should be a 12mm brass plug, you can remove that plug and black propelyne will shoot out, make sure the system is depressured before doing this.
Don't forget to Clear Caterpillar Fofklift Codes.

Randy did you find out the problem? I have the same issue.

Any ideas I have the same problem

Any Ideas I have the same problem and have replaced all the same parts still no start. When checking spark at one of the coils spark looks to be intermitted not constant like a safety of some sort is killing spark.

Swap out the EG1 relay with a known good one, I had the similar issue it ended up being that damn relay, it's under the blue cover relay box

Check the EG1 relay under the blue cover relay box, I had this exact issue, it ended up being that damn relay, swap them out with a known good one

Did the relay swap still no start and adjusted the valves (they all were a little tight) still no start.