97 GTI intermittent misfire

[andy.262]

I have a 97 GTI that I just finished rebuilding, I'm on my second tank of gas since the rebuild, so thats the "ride as normal except dont hold it wide open for more than 5 seconds" stage. It starts and idles totally fine, and runs great up to like 25% throttle (no tach, can't tell RPMs), but when I try to accelerate more from 25% I'm seeing that sometimes it does it just fine, and sometimes it feels like a misfire and will stick at that same RPM without increasing for a few seconds at which point it will sometimes continue accelerating or sometimes drop RPMs but not stall. It's somewhat rideable still, never stalls, it's just not quite right on the acceleration.

I don't think its a fuel issue - fuel selector and lines are new, carb freshly rebuily with Mikuni parts according to carb wiki. To me it FEELS like a misfire, kind of feels and sounds like on my 2-stroke sled when it has a bad spark plug and is running on 2/3 cylinders. However, it only happens at high-ish RPMs and only in the water, on the trailer it revs right up just fine every time. Replaced spark plugs, no change.

So, I am now suspecting a weak but still present spark in 1 cylinder maybe? So, either ignition coil or bad plug caps or wires? Anything I can do to test those without buying parts?

 

[mikidymac]

Running on the trailer will tell you nothing except that it runs so don't compare that with in the water.

Cut the cable tie on the sparkplug caps and unscrew the caps, yes they unscrew. Cut 1/4" off the sparkplug wires and reinstall the caps. See if that helps.

 

[andy.262]

Tried this yesterday. Unscrewed plug caps, sprayed them out with electrical contact cleaner, trimmed the wires back 1/4, screwed back on and replaced zip ties. Doesn't seem to have improved at all, still occasionally cutting out in the mid to high RPMs.

I wouldn't think that it would be a timing issue, because it starts right up and idles great, and when it does decide to go it sounds and feels great. I would think a timing issue would be all the time, not sometimes.

I also have a 97 GTS with the 720 engine just like my GTI, and that ski runs perfect. Are there any parts I could easily swap from the good ski to maybe help determine the issue with the GTI?

 

[WaterChampion]

Maybe swap the ignition coil? Maybe swap the mpem? Have you played with hotter plugs or played with the plug gap? Maybe run it on the water around idle or just a bit more for 5 minutes then kill it and check spark plug to see if it’s wet or dry, then run it and throttle up and as soon as the problem occurs pull the key and check the plug again. Could it be oil rich hesitating?

 

[andy.262]

Looks like the GTS and GTI, even though they both have the 720 engine, have different ignition systems. According to the manual, the coil on the GTS contains a CDI module and the coil on the GTI does not. The MPEM is also different between the two. So i guess i can't be swapping ignition parts from the GTS then.

I did check the spark plugs after idle, they looked pretty good. I will check them after pulling the key next time the issue happens. I did align the marks correctly on the oil pump when i put the carb on, but i guess we'll see what the plugs say, maybe it is using too much oil at the mid-high rpm range.

 

[WaterChampion]

Didn’t know that. Might not be good to swap. Maybe the coil could be on its way out. I would assume a problem at the coil before the Mpem anyway. I did do a full carb clean and rebuild once and put it back together, had issues anywhere after half throttle point and it ended up being a clog in one of the jets even though I just finished cleaning and rebuilding…. After that I put inline fuel filters on right past the water bowl catch.

 

[andy.262]

To me it feels like it's running on 1 cylinder when the issue occurs. It doesn't really feel like a fuel issue on account of its more of a "switch" kind of deal rather than a gradual bog. It either revs up past that problem RPM level or not, and when it doesn't make it past it just kind of holds at that specific RPM without dropping or gaining RPM, I think I can even hear it misfiring but hard to tell. If I keep holding the throttle at the same level it usually starts accelerating again after a second or two of holding. If it were a fuel issue, I would expect to be seeing a gradual bog. As it is, it starts right up in less than 1 second and idles perfectly, and once it makes it past that specific RPM I can hold it wide open and it runs fantastic at the rev limiter.

I pulled the key when the issue happened to check for oil fouling, and both plugs looked pretty much perfect. Maybe a touch dark, but not wet or charred.

I opened up the rear electrical box to get a look at the coil and see if maybe a connection was corroded or something. Thankfully it was dry in there and nothing really looked especially corroded. I pulled the 2 female wire connectors off the coil and tested across the 2 blades with my multimeter. I am getting around 3.4 ohms, which i think is very high - manual says it should be between 0.34-0.62. I was suspicious of that reading, since it runs so well at all other RPMs, so I even got a different meter and measured again with the same reading of around 3.4 ohms. Does that make sense that it would measure so high yet only have issues at a very specific RPM in the middle of the range?

 

[WaterChampion]

Strange, could the multimeter be on a decimal off setting? Did you check pop-off pressure when you did the carbs? My gti was not doing much past half throttle and I noticed 1 plug dry and the other fowling but it’s a dual carb

 

[andy.262]

Well, it was definitely the coil for whatever reason. I looked up a new one and noticed that the part number was interchangable with the one for the 97 SPX, so I grabbed the one off my SPX and swapped it on, and now it works perfectly. It's weird because the faulty looks brand new and the one that actually works from the SPX looks like I just pulled it out of Davy Jones Locker lol

 

[WaterChampion]

Haha it definitely looks like it came from Davy Jones Locker! I see the old one is Japan made, I wonder about the other. Congrats on getting it figured out! You ride in Salt Water?

 

[andy.262]

Nope, great lakes area here. As far as I know, this ski has only ever been in fresh water. Although, it HAS been underwater a few times... that probably didn't help...

Good point on the branding there, it looks like the "clean" faulty one may be an aftermarket coil since I don't see any branding or markings on it whatsoever. This may be another story of original-quality parts being superior to even brand new aftermarket parts.

 

[andy.262]

OK, this is a PSA to anyone reading this in the future - the cheap ~$30 aftermarket ignition coils available on eBay are straight JUNK.

I got a brand new one to put in my SPX, which was previously running perfectly. When my GTI was acting up, I took the original OEM coil from the running SPX and put it in the GTI, which had an aftermarket junk coil, and the GTI started running perfectly. That left the SPX without a coil, so I figured I'd get a cheap one for now even though it might not last as long as a good used OEM one. Boy was I WRONG. I opened the sealed package, immediately tested it with the multimeter and got 1.5 ohm resistance, factory spec is 0.34-0.62 so thats WAY higher than it should be even straight out of the box. I put the brand new aftermarket coil in the SPX, and it started showing the EXACT SAME symptoms as described above for my GTI with the aftermarket coil (I assume, it has no markings).

The brand of the coil was "Jet Ski Plus" or something like that, they are all over eBay in the ~$30 range. There are no identifying markings on it whatsoever, the listing image shows a sticker on it but the actual part has no sticker, number or anything. I am thinking that the bad coil I had in the GTI before is probably the same brand since it caused an identical issue as the brand new one.

Save the headache, find a used good coil that has the "made in Japan" sticker as seen in my photo above on the crusty one.

 

[burtshaver2021]

Thanks for sharing that information. Glad your back on the water

 

[mikidymac]

This really sucks and we see it over and over.
I don't understand why they can't just make good aftermarket parts and charge $60 instead of the junk ones that don't even work for $30.

All we can do is keep telling people to not buy the aftermarket electrical parts but at some point the good used OEM stuff is going to dry up.