Been fighting an idle issue on one of my 502's. Gen VI, Cool fuel, MPI. Top ends were completely done, about 50-75 hours ago and haven't had issues until just recently.

Took boat out Labor Day, and starboard motor was hard starting. Got it started, and ran fine on plane away from ramp. Stopped about 10-15 minutes later and the engine didn't want to idle. Stumbled and quit a couple times and at dead idle had a very "lopy" cam sound. Again, got on plane with no issues and seemed to run fine on plane.

Initial thought was maybe a bad sensor or possible IAC valve.

All temp sensors Ohm'd out within spec and matched to port side motor (good motor). Swapped throttle bodies with no change. Swapped ECM's, no change. Fuel pressure is around 40 +/- at idle, and vacuum is at 14 in lb's at idle.

Swapped ignition module at Distributor and seemed to help a little but still won't idle consistently. Almost goes in and out, but won't quite like it was before. Pull idle up to 1000 rpms and sounds fine, only now seems to run rough at idle.

Haven't checked compression yet, since I initially thought it was just a sensor. Now I'm not sure, since basically all sensors have been swapped between engines. I did end up putting a new IAC in, but again no change.

Would a leaky injector cause the intermittent idle lope? They were cleaned when the top ends were done but it has been almost 2 years.

the IAC rarely are at fault being the original GM design for MEFI efi apps. You will need to SCAN the engine, also do fuel samples and change the fuel filters. You need to do fuel pressure testing. Have you checked plugs and wires. Possible swap over from the other engine to what changes or not.

Your vacuum reading is low.

Like I said a Scan would be a good test here and possible a MAP sensor issue - I really would look into this

Other testing -- run the problem engine on a remote fuel supply to see if anything changes. At least that way you know if its an engine side problem or the boat fuel delivery side problem.

Fuel pressure at the shrader valve is around 40 at idle. Does bounce +/- a few lbs.

Don't have a scanner, but did put it in test mode and there were no stored fault codes (paper clip and a meter)

no fault codes from a winky blinky means nothing period even if you scanned the engine with a real scanner. Monitoring running parameters - also voltage - injector pulse widths ( MEFI) , and so on is the real way to diagnose using a scan tool. All inputs and outputs of sensors need to be in the spec ranges and correct voltage ( 5 V and 12 V) is what one looks at with a scan tool. The issue is one has to know all the correct spec ranges from start up to warmed up and from idle to WOT.

Unsteady fuel pressures can be an issue as well.

Leaking fuel injectors can easily be tested using a fuel pressure gauge

Also the fuel injectors can be tested with the scan tool and the correct adapter.

If need to be I can test, clean and balance flow anyone's fuel injectors for the gas side of things. Almost 9 years experience.

Thanks BUP. To expand a little I did do a bit more than check for fault codes. Checked the voltages and OHM readings where I could per the manual in the “without scan tool” section. Was trying to check as much as I could without a scanner since I didn’t have one handy. Apparently, I'm at the “need a scan tool” level. Haha. I hope to borrow one over the weekend and start to get some more info.

Just because you cleaned your injectors 50-75 hrs ago doesn't mean they are clean today. Ethanol is wreaking destruction on older boats gas lines. Eating them form the inside out. So unless you replaced ever inch of fuel line in your boat don't rule out injectors. Your issue sound fuel related to me.

Unclear weather you changed the Dist Cap and rotor?. I had that problem and it fixed mine but I am guessing you have already done that.

Chris

Lopy idle after a run and sitting for a bit, would hint toward slight vapor lock issue. Low vacuum, fuel pressure not steady, top end redone less then 75 hours, maybe a vacuum leak and or fuel pressure regulator.

<br /><br />I thought the same as well.<br /><br />I would try coil and cap and rotor and plugs. Also ohm test the spark plug wires. My point is the issue to find out exactly is it fuel related or ignition related. Somewhat hard to say 100 percent fuel related without eyes / hands / ears and inperson testing.

Just an FYI. I had a Indmar 5.7 with a fluctuating idle rpms this summer. At first --- For sure thought it was the TPS causing this - and really was 50 / 50 on the IAC. The IAC was the GM style app and mounted to the TB. Like I said more times than not the IAC is not an issue for that exact set up. The tps on the other hand can be an issue inwhich can or will fluctuating idle rpms.

Anyways, I did cap and rotor - plugs and wires cause all that was original from day one and needed it regardless to start out with - then move forward with further testing . ( EST ignition set up all stock app -- 2003 model wakeboat - never had a tune up period and all ignition side was never changed so it was going on 15 years old with about 500 hours on engine.

After the tune up, it still had a fluctuating idle - but seemed even worse than before. After thinking about it, I decided it prob was the est ignition coil. Yep the coil fixed the issue. Weak spark causing for the idle rpm issues plus the efi system / ecm compensating from the received inputs and basically trying to correctly compensate the signal outputs for engine running (for a short jist of an explanation )

Basically in short words --- my point here you know how many people would have thrown an TPS and IAC and fuel related side parts at this plus messed around with the throttle / cable / plate and still have the same issue ? Prob most everyone. So sometimes sensors and fuel is not the issue. But how the ecm / engine receives input information and send outputs can be the real issue. Voltage and grounds are very important and in the mix as well for all engines.

Worked on the motor again this weekend and had a DDT. Ran on both motors and didn't see anything glaringly different between the two. Fuel flow rates were varying between .77-.95 with injector pulses around 3.41 +/-. IAC readings were different between the 2, but my guess is the bad side is compensating for the weird idle pattern. MAP sensor readings were around 18 on both motors. Fuel pressure matched on both also. And the bad motor did not bleed down with the key off.

No codes in either. As soon as I pull the bad side up to 1000 rpms, it goes away completely. Back off to dead idle and it comes and goes.