Was out for a 30 minute drive on Saturday, and during the second half of the trip, I was getting a hesitation that felt like a single cylinder misfire. It would only happen under load (i.e. during acceleration), but would run smoothly at while cruising at constant speed.

It sounded distinctly like it was coming from the right side of the car. I assume it was related to the fact that the car was at temperature after 15 minutes of driving. The car also seemed down on power.

Got home, connected the Durametric, and nothing. No error codes found.

Sunday, I took the car out for a drive with the Durametric connected and datalogging. About 30 minutes of spirited driving and the car is running fine. No misfire, running normally, no error codes. Looked over the data, and didn't see anything unusual, but I'm not sure what to look for.

Some other miscellaneous information:
1. My dash coolant temperature is running a bit hotter than normal. The needle is usually between the 8 and 0 in "180", and now it's in the middle of the 0.
2. It rained for 6 days straight prior to Saturday, with 100% humidity. During that time, the car sat in the garage. Condensation somewhere?
3. I did the "orange cap mod" about 3 weeks ago, blocking the air intake connection to the airbox resonator. (But my cap isn't orange, it's grey. Does that matter? )

Not sure what to check next. The car is running fine today, but I'd like to get to the bottom of this mystery.

 

Bad batch of fuel maybe? Just thinking out loud.

 

I had the same thing under load. Ended up being a clutch. See my thread on the 996 I'm doing. It felt like a small misfire at first. Then it felt to me like a vario cam thing. Then I kept driving and it turned into a chatter. It never really slipped. After looking at the flywheel it was slipping causing the marks on the flywheel which caused the chatter. How many miles on on the clutch? I HAD a light weight flywheel and GT clutch. I am just switching to stock duel mass and stock clutch. My leg will thank me.

Robert

 

Before you go ballistic and repair/replace everything on your engine, run a tank with Techron. Could be just a little bit of injector fouling, bad gas, a hiccup. Get a good highway run in. BTW, the color of the cap means nothing and blocking off the resonator chamber is not the problem, if there even is one.

 

Less than 20k miles on the clutch. I know the feeling when the clutch is going, but my current problem is different.

The idea of bad gas or water condensation had occurred to me, but I'm not sure why the problem would show up after the car had warmed up, then just disappear the next day.

BTW, drove the car in to work this morning and did not experience any issues. Thanks for the suggestions. Any other ideas?

 

Coil pack? Rain might cause an spark ark also if one of your wires are bad. With out an CEL its hard to tell.
Robert

 

How old are the spark plugs and coils? I would check the TRA, FRA, MAF reading, coolant temp reading, and misfire values in Durametric. If the engine continues to run smoothly and still no codes, you may need to wait for the hiccup to reoccur in order to diagnose it further. Also, it's good to have an OBDII scanner ready so when it happens, you can plug it in regardless if the CEL is ON or not.

 

Plugs are 3.5 years old (and about 17,000 miles).
Coil packs are of unknown age, but more than 4 years old.

I assumed if plugs or coils were bad, it would throw a code?

I will check the above Durametric values (recommended by Ahsai) when I get a chance, but my only OBD scanner is via my laptop, which I don't usually keep in my car.

Remembering one more thing based on occb2's comment above... Just before this problem first occurred, I drove through a 0.5" deep puddle that splashed as I drove through it. Perhaps this was highlighting a bad coil pack or wire?

 

With no CEL and no error codes it was not a misfire.

My WAG is the AOS is going out.

Less likely but a possibility nonetheless is the fuel pump is acting up.

I would not replace either item though on just my WAG.

You can try the Techron as DBJoe996 suggested. While it is a good idea it probably won't help but you will feel like you are doing something positive.

Use the Techron according to directions. I like to change the oil/filter after the last tank of Techron treated fuel is gone, or nearly gone, down to say 1/4 tank.

If it is the AOS (or the fuel pump) you'll know at some point.

Oh, if at any time the engine really starts to misbehave, exhibits a varying idle/hunting idle, misfires (with a solid or flashing CEL), make scary noises, smokes, shut off the engine ASAP and flat bed the car to a qualified shop for a professional's diagnosis and advice.

 

The AOS was replaced at about the same time as the spark plugs, about 3.5 years ago. (still could be failing again I suppose)

Regarding the fuel system, I do have a new fuel filter in hand that I have not got around to installing (since they are a pain to get to on the '99). Not sure if that's worthwhile to do ASAP.

 

The CEL will be tripped only after the misfire counters exceed some thresholds. Maybe it happened so briefly that it didn't trip the CEL. Hooking up the Durametric and monitor the misfire counters will be your best bet to catch misfires in real-time. The fact that it happens under load only points more to ignition and fuel delivery (I know that doesn't help much). If yours a MkI, when was the fuel filter replaced? I would still suspect coils and plugs at this point given this happened after you drove through a puddle of water.

 

Yes, it did only happen under load, but for the 15 or so minutes it was going on, it was repeatable. (i.e. depress accelerator to experience the "misfire" issue)

The car is a '99 C2 Cab (MKI) with 70k miles, and as far as I know, the fuel filter has never been replaced.

 

I agree then the CEL should have been tripped. That sounds very much like a clogged fuel filter but of course nothing can be confirmed until it's tested properly.

If it's original, I think it's good to replace the fuel filter as a PM anyway.

 

Agree with Ahsai. Time to think very seriously about replacing the fuel filter. They have a limited amount of room to hold any debris they catch and based on my experience with other fuel filters in other cars do not like any water in the fuel at all.

(My 02 Boxster's in tank (at the pump intake) fuel filter was immune to water, not that there was any water sign at all present when I had the pump replaced at around 200K miles but there was a lot of fine particulate trash in the filter mesh. There was plenty of mesh area though so the fuel pump was not being starved for fuel. An inline fuel filter though I think would have been plugged up long before with that much debris. BTW, my 03 Turbo has a small inline fuel filter and I have it replaced on schedule, whatever that is... The SM looks it up when I had the last one done and if it is time to change/replace I have a new one installed.)

 

Thanks for the advice gents. Another day, and still running normally.

At this point, I'm planning to change the fuel filter this weekend, fill up with some fresh gas, and then wait and see what develops. If the issue returns, I'll connect up the Durametric and see what I can learn.