Hey got the motor back in and fired it up. Ran her around town for a while then after 20 some minutes it started acting up like before. It cuts out constantly. I bought a new pcm before the motor came out, but it is still doing it. Rode around with the snapon before and it said it was losing sync signal with the pcm. tested the grounds and power to the pcm and all is good.

I am moving to texas in a week so I need this fixed now. Any imput before I have to take it to the dealer and spend all my moving money?

 

Just thinking out loud here

dont think its the fuel pump yet

crank sensor would not cause pcm shutdown, but i have a new one in the garage so I will replace it tomorow.

Ignition swich could be going bad. will probe it with a test light tomorow and drive it, see if it blinks when it cuts out.

 

I don't know what your previous problem was but, a bad crank position sensor will cause it to shut down. As well as a bad cam position sensor, fuel pump, relay, ground...

 

You need to re-time your cam with a DRB III tool to reset your sync. The offset value is most likely too large thus causing the loss of sync between the CKPS and CPS / Crank and Cam relationship.

Whenever changing engines or cams or even the distributor, it must be reset. Bring it to a dealer and have them reset the offset value with a DRB III tool.

That will take care of that issue.

Chris

 

Thanks but the truck ran fine for 2500 miles on the rebuilt motor

hoping its the ignition switch

 

The way it was worded, it sounded like you just got the motor in and only drove 20 minutes. What is the history here? This seems like it's continued from another thread as noted here: What is the symptom/s?

Just read your PM. Ok, it works like this. -CKPS: The PCM interprets the sensor input to determine the crankshaft position. The PCM then uses this position, along with other inputs, to determine injector sequence and ignition timing.

-CPS: The PCM uses the signal from the camshaft position sensor to determine fuel injector sequence. Once crankshaft position has been determined, the PCM begins energizing a ground circuit to each fuel injector to provide injector operation.

Both of those are used to control the timing. All EFI vehicles have what is called a sync offset value assigned at the PCM level. It's a parameter value that is preset at the factory and is used to fine tune the cam relationship to the number one piston at TDC. It is directly correlated to the CKPS and CPS sensor voltage at the PCM input. After changing a distributor, cam, crank or an engine that is not the same as OEM one, the PCM sync value must be reset to as close to zero as possible and, the only method of doing this is with a tool that can write a new value into the PCM program. This can be done with either a DART or DRB III tool. These tools are around 8-10K so, have a dealer do this or, anyone that has the tool AND knows how to properly re-time the cam.

This change is an electrical change only and, it's done at the PCM programming level by simply timing the CAM first so that it's dead nuts to TDC (some also fine tune it a few degrees advanced depending on application e.g. racing engine) and then reset the sync value to ensure that timing does not go out. If this value is too large, or, after an engine wears in, this timing relationship between crank and cam changes and puts the value out even further. As the chain stretches, the cam wears in, the diz gear wears in, etc, the timing drifts and, so does this sync value.

If it has gone way out, I believe that there's a safety / failsafe mechanism that cuts the engine out to prevent catastrophe. Now, I don't even know what year your truck is our what level OBD your running.

I would like to get some history to become more aware of what the issue is. At this point, the only thing that I have to go by is the loss of sync which, when I hear loss of sync, this is the first thing that comes to mind other than a buss issue in which, the power module to logic module problem.

 

built a 408 2500 miles ago with small problem here and there but they were all iorned out. Then on a seperate problem we were given the wrong rings in our stroker kit and had to replace them. The problem with the fuel sync started 50 miles before we parked the truck to replace the rings. Now we bought a new pcm and flashed it when we put the motor in with the right rings. We fired it up and drove it 20 some minutes to seat the rings. then it happend. started cutting very bad. Just like before we yanked the motor with the old pcm. (the snap on scan tool says that it is losing sync signal with the pcm) truck cuts in and out very fast. like it is losing power to the pcm

 

Ah, yes, I remember reading that. Ok, so, we're looking at a fuel sync issue even before putting new rings in. I consider that very fortunate to have gotten that amount of miles out of it without ever re-timing the cam and resetting the sync value at the PCM.

So, once again, you had to tear down the engine, put new rings in and put it back together. So, this time, it sounds like your cam is further out from when you originally put it together thus worsening the original problem.

What year OBD do you have installed? What year is the truck / wiring harness? Are you running the stock OEM harness or a modded / custom harness?

Reason I ask is, when I start going through schematics, I'd like to know so I don't spend a ton of time looking at them only to find out later that your truck is not wired the same as OEM.

Chris

 

I dont know what else to describe besides it cuts out very fast. Dash lights blink. It cuts out soo often it jerks quite alot . And of corse like I said already computer says it loses sync signal. have replaced almost eveything. When the motor was first built it ran rough, Krc performance had me index my distributer gear form pointing to the #1 cylinder to almost 11:45 position. Seemed to run better. But not good until I had my iac reset by the snapon tool. And like I said it seemed to run pretty well untill just recent.

 

96 ram stock wiring harness (spliced in different injector connectors) had remote start that just stopped working recent also. Refurb pcm flashed with my sct with a 408 tune. Weird thing is idles fine. not untill you drive it and get the rpms up to about 1400 or so