Rough Running ZZ502
 

OSO!!

I'm struggling and looking for suggestions to get the port engine in my 42’ Sportfish to run smoothly below 1500 RPM, and at idle.

The boat is powered by a pair of ZZ502 short-blocks installed last spring, paired with new AFR 265 heads and Holley HP TBI EFI. The starboard engine runs flawlessly, while the port engine has a rough idle and a noticeable miss up to 1500 RPM. Once throttled up, both engines run smoothly above 1500 RPM, make solid power, and burn equal amounts of fuel at cruise (3500 RPM).

What I’ve done so far:

• Replaced intake gaskets
  Smoke tested intake system – no leaks found
• Swapped throttle body and distributor with the starboard engine
• Replaced spark plugs
• Replaced ignition coil
• Re-adjusted valves
• Verified compression (175–180 psi across all cylinders)
• Confirmed both engines are running the same tune
• Temporarily installed a carburetor with distributor disconnected from ECU

Above changes had no effect on my poor running engine.

Could this be a lifter issue? Has anyone experienced problems with GM lifters in similar setups? I usually work through these kinds of problems myself, but this one has me stumped. I don’t have a problem buying a set of lifters if you think it is possible they could be issue.

Fingers crossed that the collective wisdom here on OSO can help me figure this out—before I pull out what’s left of my hair!

Thanks,
Steve

Can you tell which bank has the miss ?

1357
2468

try pulling 1 plug wire at a time to see which cylinder has the miss? The plug wire that does not appreciably change the running condition is probably the cylinder that's wonky.

I suspect I'm getting a miss from each bank—at least, that's how it feels. It's hard to test by pulling plug wires because the exhaust exits below the waterline at idle, causing a lot of bubbles to smack the bottom of the boat. This makes it difficult to tell whether the sound is just water turbulence or an actual engine miss.

With good compression across all cylinders it seems lifter weakness is questionable, but the fact that it cleans up around 1500 revs points back to oil pressure or weak spark at lower rpms ?

sounds like a stupid question but what plug wires are on it? just finished battling a problem sniper setup on a 455 olds that kept having emf problems. it idled strange but ran good in the upper range. found that all 8 of the plug wires at the distributor end were assembled with only about an 1/8" of core exposed and not wrapped up and under the terminal end so I had about 3/16" air gap from the terminal to the wire. I never measured the ohms of the wires as they were new MSD wires but now that I fixed the wires it is all working like it should and idles way better.

It has great oil pressure, and wires, cap, and rotor are Sierra Marine. I've had great luck with them over the years. I have not tried the wires from the other engine. I will do that!

Well, swapping cap, rotor, and wires from other engine did not change anything.

Not sure where to go from here. I'm seriously considering installing a set of lifters. I don't know what else to do.

I have readjusted valves twice using 1/2 turn from zero lash. Would more or less preload be worth trying?

Id say your preload is correct as long as you are on base circle of the cam for that cylinder, I run them in the firing order......TDC #1 then 1/4 turn of the engine set #8 1/4 turn and so on as long as they are standard firing order ?

These were factory assembled plug wires? Or custom assembled??

Thanks

So how do you know there is a problem at all??

Pulling plug wires is your #1 troubleshooting process for this type of problem. Eliminating a poor combustion event and offending cyl.

Otherwise, engine stethescopes or even Doc stethescopes work well to isolate and home in on problem locations.

What are you running for spring pressures? The ZZ502 factory rollers don't like a lot of seat pressure. Anything at or above 150# can cause issues.

From your description, it sounds like a collapsed lifter that never recovered. But you set the lash @ 1/2 turn with no noticable issues.

Sierra Marine Wires Both Engines. They were installed new with engines

I think it is a random missfire @ idle, then turns into a hard miss when getting on plane, then runs perfect over 1500 RPM

I don't remember pressures. AFR used specs provided by Teague Marine to match their springs to Teagues cams


Thanks for the replies!!!!

Check...so you're not running the stock ZZ502 roller cam?

Inside the box of prescribed use, the GM roller lifters were relatively reliable.

The AFR 265 BBC Heads baseline Hyd Roll springs start at 175lb seat. PAC-1940 469/in rate. Puts it around 430lbs for a 0.550" lift.

As you mentioned, Teague may have spec'd the springs, to a more suitable rate for their cam.


***ZZ502 springs setup for 140lb seat for crate eng.

the ones I was dealing with were assembled by MSD so mark down another MSD failure to add to everything else they are screwing up lately.

Steve H, what ECM's are you running...... so this engine has never ran well ?

No, this engine has never ran well. Running Holley HP ECUs. I forgot to mention above that I did swap ECUs with no change in port engine.

What brand distributor are you running ? Have you pulled it and inspected the drive teeth? Had a similar situation years ago with an engine

have you pulled the plugs right after the mis?almost sounds like reversion.sure you don't have any exhaust leaks?

Sounds like a vac leak to me. Maybe an "internal vac leak"??
Could connect a vac gauge to verify.

What's fuel pressure when the engine is running poorly?

I'm using GM small cap HEI. I have replaced p/u and module with OE GM parts, drive gear looks new.
I have pulled plugs, #8 and #5 are slightly brighter than the rest. No signs of exhaust leaks
I thought the same zz. The first thing I did was replace intake gaskets, and smoke test. No vacuum leaks! I Vacuum gauge reads in the yellow(15) inches? Needle moves minimally.
Fuel pressure reads about 20 PSI-Holley HP spec for TBI.

Last night I readjusted valves (again!). Used 1/4 turn past zero lash instead of 1/2 turn. Engine runs better, but still not quite right. Next step is double check timing ensuring that
actual timing is matching commanded timing.

Thanks everyone for helping me out!

Any ideas on what would have changed with less lifter pre-load? Maybe one of the lifter cups was sticking when further down the lifter bore?

What oil grade are you using and what pressures below 1500rpm?

I was planning to use the BBC factory lifters, but maybe I'll go with the Johnsons. They're just quite a bit up here, landed north of the 49th, if only running a relatively mild cam.

Forgive me for pointing out something you probably already know. Hydraulic lifters can be a little tricky to adjust. By the time you get to the rocker there is a chance the lifter has bled down already. So it can be difficult to tell where the 0 lash actually is. Has always been my preference to do hydraulics while the intake is off so I can see the plunger.

Another thing - I always use a remote starter switch to crank the engine. I don't bother looking for or trying to keep track of TDC. Make sure the engine is not going to start. Key off, kill switch and disconnect fuel pump.
Adjust intake when exhaust starts to open
Adjust exhaust when intake finishes closing

Have you hooked up a vacuum gage. Does the needle bounce. Bounce between what two trains and how fast? You can look up vacuum gage reading online to help you diagnose.
As for pulling plugs, hook up s tack and record rpm drops for each cylinder disconnected.

I'm not sure a smoke test would alert you to an internal vac leak. Fortunately it easy to check. Remove PCV, plug all valve cover holes, rmv oil fill cap and place the palm of your hand over the oil fill hole (best to do on a cold eng). You should feel crankcase pressure start to build at idle. If you feel vac pulling on your hand, you have a leak where the intake meets the head/s but on the bottom (crankcase side). Also if you have a PCV Valve, could try pinching off hose (or pluggng it) and see if it smooths out. Another easy thing to try is partially blocking air coming into throttle body which will enrichen mixture and if it's lean, it should smooth out.

Would be good to compare idle intake manifold vacuum readings between the 2 engs. 15" may be normal for your cam, or may be a little low. Hard to say.