Misfire after head replace

[blue62]

Originally Posted by McSpooney

So I think it failed the vacuum test. I guess googling this test makes me think its a sticking valve and or bad valve guides... but maybe It would be best for you to see the video. The needle starts out a little wobbly... then goes crazy flying back and forth from 22 to 16 in hg... eventually stabilizes. It seems to do it when the car is idling very low... at 680 per durametric, with a target idle at 680. One caveat though: I ended up tapping into the right behind the throttlebody where it branches off for the AOS and that fuel tank purge valve. I have never seen a fuel pressure regulator on my car so couldn't really tap there. Keep in mind mine is 2003 3-chain, doesn't show a regulator in the katalog.

I also did the vacuum test to see if the cat was the culprit... vacuum didn't seem to drop when holding 2k rpm. I also visually inspected the cat... with a little boroscope though. It looked fine.

Putting in a link to a video of the vacuum gauge. The needle starts freaking out at around 25 sec, but eventually calms down at around 3 minutes. Engine was warm already from previous running, but only on about 1 minute before i started the video.
https://www.youtube.com/watch?v=jzao8NScuJE

I'm a little confused if this is a valve issue. I really hope it isn't. But I roughly felt each valve as I installed... clearances felt consistently good at install. Unless this is an issue with the intake valves, but those appeared to never have been bent and just seemed to be 100%

I'm actually a little familiar with verifying cat function. I would typically look for the post-cat O2s to see if the wave function flat-lines after the cats get to temp and I'm cruising. Right now I'm using my desktop for durametric though lol. I'll have to devise a way to check the sensor reading while the car is moving. The exhaust smells pretty bad btw. Makes me think cats aren't doing so hot, but also makes me think maybe the injectors.

I replaced the ignition coils... and as expected... that was a waste. Car runs exactly the same. Maybe fuel injectors?

So a couple of things.
First you should really hold the gauge in you hand or find a way to isolate it from the car. I know you laid it on the car so you could video but the needle will pick up vibrations from the car and skew needle action.

Second: video or post a picture of your hookup so I know it is correct.

Third: It is great the way you tested just letting it idle and giving me a good long view of the gauge.
Your probably the first person to do it correctly. LOL

When you research vacuum testing info on the web (youtube) you have to remember most all info is for testing "single cam" engines.
The Boxster is a four cam so needle action (if any) is going to be a little different and much harder to interpret.

So the bad news:
Looks like you have a valve train issue.
Possibly one or more lifters acting up.
I think I hear a ticking sound in your video but not sure.
The reason I think you have lifter issues is from the needle action.
If you watch the needle it is moving in a very specific way.
it is not bad at first just drifting between 20-21 that is not bad.
Then it goes crazy as you say but in a very set way.
Then it settles down again after a while.
Like a lifter or valve sticking.
Then correcting itself.

Please remember all this is an educated "guess" at best.
Diagnosing issues remotely is very very difficult.
But what I am seeing points to valve train issues.

You could do the test again with the Durametric hooked up and see if your misfire count comes and goes when the needle on the vacuum gauge starts acting up.

Also when the needle starts ticking go to your exhaust and listen you will probably hear a difference in your exhaust.

So run a fuel trims scan with your car fully warmed up and lets see your RAKT and FRO's FRU's what ever your other fuel trims are.
Do the RAKT at idle.
Do the other fuel trim at 2000 RPM.

For Cat function you need to get the car good and warm like a 20 minute drive.
Then hook up the durametric and just hold your RPM at 2200-2500 for 60-90 seconds and watch the post cat voltage. if everything is good post cat voltage will stay above .450mv best to see it around .750mv mostly a flat line on the graph as long as you hold the RPM steady.

[McSpooney]

Originally Posted by blue62

So a couple of things.
First you should really hold the gauge in you hand or find a way to isolate it from the car. I know you laid it on the car so you could video but the needle will pick up vibrations from the car and skew needle action.

Second: video or post a picture of your hookup so I know it is correct.

Third: It is great the way you tested just letting it idle and giving me a good long view of the gauge.
Your probably the first person to do it correctly. LOL

When you research vacuum testing info on the web (youtube) you have to remember most all info is for testing "single cam" engines.
The Boxster is a four cam so needle action (if any) is going to be a little different and much harder to interpret.

So the bad news:
Looks like you have a valve train issue.
Possibly one or more lifters acting up.
I think I hear a ticking sound in your video but not sure.
The reason I think you have lifter issues is from the needle action.
If you watch the needle it is moving in a very specific way.
it is not bad at first just drifting between 20-21 that is not bad.
Then it goes crazy as you say but in a very set way.
Then it settles down again after a while.
Like a lifter or valve sticking.
Then correcting itself.

Please remember all this is an educated "guess" at best.
Diagnosing issues remotely is very very difficult.
But what I am seeing points to valve train issues.

You could do the test again with the Durametric hooked up and see if your misfire count comes and goes when the needle on the vacuum gauge starts acting up.

Also when the needle starts ticking go to your exhaust and listen you will probably hear a difference in your exhaust.

So run a fuel trims scan with your car fully warmed up and lets see your RAKT and FRO's FRU's what ever your other fuel trims are.
Do the RAKT at idle.
Do the other fuel trim at 2000 RPM.

For Cat function you need to get the car good and warm like a 20 minute drive.
Then hook up the durametric and just hold your RPM at 2200-2500 for 60-90 seconds and watch the post cat voltage. if everything is good post cat voltage will stay above .450mv best to see it around .750mv mostly a flat line on the graph as long as you hold the RPM steady.

So I got some time to work on it... but not a lot. I had tried holding the gauge in my hand and also resting it on the car... didn't seem to make a difference, so I just filmed it as you saw. Attaching image of the setup... hopefully you can make it out... its on the hose that leads to the AOS. Probably hard to see with the shifter cables in the way.



I ran the engine at idle 2 more times and drove it for like 20 minutes the other day. Honestly it's just been idling, so i don't think that the other fuel trim values are up to spec yet. After that 20 minute drive though, I checked o2 sensor responses and the cat on the side of the new head does not seem to be working... Honestly back when the head blew, it was running for like 20 minutes (between having to move it around and trying to diagnose the issue) while it was pouring out coolant smoke out of the exhaust. So maybe the cat is dead. I think I'll try and test the cat for blockage again after driving it around then maybe swap back in another set of cats I have.

So I'll and drive it again before I give you fuel trims, buuuut... the vacuum gauge drama out not seem to correlate to the misfires. I got the misfire counter to run up to 27 on cylinder 4 (sometimes one cylinder, often 4 or 6 will just shoot up in count) but the vacuum gauge was very steady. I also had the gauge freaking out and the misfire counter was barely moving. I'm hoping that's good news.

I also want to note that when i drove it around... it drove great. Felt smooth, had decent power. I think maybe off throttle it seemed to burble a little... that's about it. I think I hear it burble sometimes at idle, but it's super hard to tell. The burbling does seem like maybe its connected to the vacuum gauge drama, so that ain't great.

I think I'll try one of these engine cleaners soon.

[blue62]

As near as I can tell your hookup is good.
The engine also pulled good vacuum before the needle action started.
But the needle action points to a valve train issue.
Something that would come and go.... like lifters sticking.
Also if you have burbling in the exhaust sound that comes and goes with the needle action then that says something is going on with the valve train.
Something that would come and go.

[McSpooney]

Hey all. Just to followup. I don't like leaving my threads open-ended... but I forgot to close this one out. The problem was actually the very first thing I thought it was. Mechanical timing was off.

I redid that vacuum test in a couple different locations, including off of the brake booster hose inlet. The vacuum gage needle didn't do that shaking thing again. I think it was shaking because I had been taking the reading from right behind the throttle body. Maybe the throttle body was doing some pulse width modulation that was causing the needle to shake.

The problem was two-fold: the durametric said that my cam deviation was damn near perfect, but somehow I'm 99% positive it was off. Also, the car has an underdrive pulley, so it was hard to set mechanical timing.

I honestly did mechanical timing like 4 times in the beginning of this problem to get the two banks within half a degree per durametric. Over a year later i tried again. I moved cam timing for bank 2 over 2 degrees in one direction. I saw the misfire problem get worse, but in the exact same symptoms, just worse. So i moved it 4 degrees in the other direction and then misfires completely stopped. The whole time, the cam deviation values on the durametric didn't change. So it says it here on other threads in this very forum, but if you have an underdrive pulley and need to set mechanical timing.... good luck. Maybe there's a better way to set the mechanical timing than guess and check.

Hopefully someone else finds this info useful.