blippy

Hi guys, hoping someone can shed some light on my issue.

Over the past couple of months, I've been having, what I think is a misfire issue.

Car stumbles at idle, but ONLY after a minute or so and until the engine warms up. After the engine is warmed up, 95% of the misfires are gone.

This has been progressively getting worse. At first, I didn't really think much of it, but in the past couple weeks, the first 10 minutes or so after startup, the idle tends to be very rough. Engine visibly shakes when watching it idle.

I've never had a CEL, but decided to plug in a scanner anyway and saw that I had a pending P1399 (misfire?). I've been watching for a CEL to register, but to no avail... only a pending P1399.

Fuel trims show negative values during the rough idle (hovers around -5 or so). When the car is driven for a sufficient amount of time, fuel trims hover around 0 and the rough idle is gone.

Plugs are OEM, changed about 35k ago. Pulled the plugs the other day, and they look to be in good condition overall(light-ish brown, gaps looked good)

Car exhaust does smell rich and I think I've been noticing a loss in MPGs. I am no expert, so I am not sure, does the negative STFT values point to my car running rich? Where should I start? Possibly coils? Valves need adjusting? Map has been whacked and zip-tied. Running a Berk HFC if that helps.

Thank you all




Update as of 1/29 AM.
No pending codes this morning, the p1399 disappeared this morning. Car ran pretty much smooth except for a little hiccup when I was in 3rd gear, accelerating at about 2.5k. I glanced over at the scanner and saw a weird jump in STFT to -86 or something like that.

Idle was smooth during startup. MAP readings showed 9.4 and STFT's still were a bit negative.
After warm up, MAP readings lowered to 8.4 (I'm guessing because RPM's stabilize?) and STFT's still showed negative (around -7), but once in awhile, would jump to double digits, -13 or so (still no misfire or rough idle!) Weird. Will probably try again this afternoon during lunch.

s2000maniac

If its only p1399 code then I'm willing to bet that you got dirty injectors. First try a good fuel system cleaner like redline or BG 44k. If that doesn't do then take out the injectors and send to a company like injectorx and they will professionally clean them.
What year and how many miles on the s2k?

silvermonster

what year is your car? are you able to get a MAP reading? if your map reading is showing high at idle then I would take a look at your valve adjustment. When was the last time you did your adjustment?

I was having the same thing, running rich and couldn't find out the cause of it and VA fixed it good luck

blippy

02, about 132k miles on chassis. Engine was reman'd under warranty at 40k by the original owner. Will try that and report back (was about time to dump a bottle in anyways)


Last adjustment was quite awhile back, probably 60k? I think I can get MAP readings, I'll check tomorrow. Thanks
Any reason that the rough idle mostly disappears after getting the engine warmed up though? Still confused about that part.

s2000Junky

I'm thinking the valves could be out of adjustment. Metal parts expand and change as they get up to temp and if you have a valve or two that are just outside spec enough to cause this issue but fall back in enough when heated up, could be your culprit. It could be something completely different as well, but I think this is going to be one of your top 3 candidates to look into. Do the easiest first.

s2000maniac

See the thing is my buddy is having the same exact problem. We did compression test which came out good and also did a valve adjustment. Non of those helped. We also tried two bottles of fuel system cleaner which didn't help either. He car is throwing is multiple mis fire codes and p1399. So I installed my s2k coil packs on his s2k and it started sounding better and all the codes went away expect the p1399 code. So that only leaves us with injectors and we gonna take those out and send them in to get them professionally cleaned. His s2k only mis fires at idle but other thang that it drives good, with no loss of power. The car has 185,000 miles so I'm thinking one of them previous owners must have used a lot of 87 octane gas instead of 93. 87 it's a lil dirtier and the s2k doesn't have a fuel filter except at the injectors. So we'll see how it goes. We all also ordered 4 brand new 04-05 coil packs.

blippy

I'm thinking the valves could be out of adjustment. Metal parts expand and change as they get up to temp and if you have a valve or two that are just outside spec enough to cause this issue but fall back in enough when heated up, could be your culprit. It could be something completely different as well, but I think this is going to be one of your top 3 candidates to look into. Do the easiest first.
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That sounds reasonable. I'm overdue for a valve adjustment anyways so I'll probably pull the cover in the next week or so to take a look.


Odd thing is, my pending p1399 comes and goes, so no CEL registers.. which sucks because I can't pull any freeze frame data

blippy

Weird question, but could a clogged exhaust system cause these conditions?
Reason I ask is because when I did switch out from oem cat to the Berk HFC (old cat failed), I thought I noticed the backfires disappear for a couple weeks. Could it be that parts of the old cat somehow broke apart and wound up in my stock exhaust somewhere, restricting it?

s2000maniac

If the cat was broken inside it would make a rattling sound at idle.

blippy

Good update for me

Dumped half a bottle of Redline SI-1 into a tank today and the car ran much better! No misfires felt, though the code was still pending (didn't reset it, hoping that a CEL would come up)
I guess it was injector problems after all? Guess I'll be looking into replacing my injectors when symptoms return.

Going to adjust my valves either way sometime this week when I get the time. Hopefully this is the end of my misfire troubles


Thanks all