rhill225

I am running an LM7 5.3 motor in my hydroplane race boat. It's a stock motor aside from a decent camshaft to open it up. Its direct drive to the prop shaft running off the accessory side of the crank, no gearbox the flywheel is exposed. I have been running the motor as an experimental motor in APBA races in the E350 class for two seasons. It all works great; the motor makes great power but I have been fighting a surging issue at or near 5500 rpm. PCM rev limiter is set at 7000rpm. I have deleted as many sensors as possible off the PCM to trouble shoot the issue, so I am running on just the MAP and O2 sensors along with other required sensors such as crank and cam position sensor, knock sensor. I have added a fuel surge tank, I have plugged off vacuum lines from valve covers, I have cleaned ground wires and positive connections. Replaced the entire wiring harness. I am running a bluedrive to collect data and this past weekend I had the same surging issue and looked at the data to find some really weird and random RPM spikes. Recording data at .05 seconds per I saw jumps in RPM register 5500rpm jump to 13,700rpm followed by 8000rpm and then back to 5300 rpm. There were many of these odd readings with 13,700 rpm being the most extreme. We all know the motor never turned 13,700rpm but mystified as to why the readings are so weird. O2 sensors are reading between 0.4 to .9, timing seems to be adjusting okay with rpm. Any ideas? I am about to yard the stock PCM in favor of a Haltech Nexus Rebel but anxious that this may not solve the problem either.

NicD

iTrader: (7)

Could be a PCM issue, but I highly doubt it as the OEM computers are the most reliable out of all of them that you could choose to run the engine. A new crank position sensor is easy and cheap enough to start with, the stock PCMs are cheap enough to replace though too assuming you have the ability to write your existing tune to a new one. Those are really the only two things I can think of that would cause a spike like that in engine RPM. I suppose the crank could be moving around enough to have the reluctor not line up with the sensor anymore either but I would suspect there would be other signs of an issue as well. Check crank fore/aft movement for possible thrust issues.

rhill225

Thanks NicD - I will throw a new crank position sensor in and also a cam position sensor just for good measure. It's interesting that you mention the crank moving around enough to have the reluctor not line up with the sensor. Running a prop shaft off the end of the crank shouldn't be a problem so long as I am not side loading the crank. I use a thrust bearing to keep the shaft in alignment but defiantly worth checking out the crank movement and everything related. I have been losing a little oil around the crank seal on the coupler. Good information on the PCM reliability. The real challenge is time to test anything out. This boat can only run on a race site's so my test time is limited and extremely valuable.

TurboS10

iTrader: (12)

I had the same issue randomly on a 02' v6 w/411 pcm at lower rpms but above idle. Finally did it while I was logging. Changed cam and crank sensors and it went away.

Doug G

Just spit balling...but could a connector maybe not making good contact and with vibration arcing ?

Weird issue.

LilJayV10

iTrader: (39)

Does it do it the same RPM or load every time?

What PCM are you using?

How many miles on the engine. I have seen engines with excessive thrust bearing wear, that allows the crank to walk, that caused strange issues, but normally at high RPMS.

Do you have a datalog of it doing it?