3.2 Hesitation - I'm going nuts throwing parts at it! Help me look for vacuum leaks!

[Dubbin']

Originally Posted by cgarr

This would be one I would want to stuff an LM1 up the tailpipe and see what the mixture is doing through out the rpm range: I would think a vac leak would cause idle problems more than WOT?

You didn't happen to spray any type of contact cleaner around or inside the computer plug?

I agree that checking the AF would be nice... One of the reason I have not been driving it much from fear of it being lean. I just figure that even if I were to test the AF and it was off - it would not tell me much about what actually is going on... I did not spray anything on or around the ECU. I just opened it up and looked at it. It looked perfect.

From what I've read... Vacuum leaks create ALL kinds of problems...

Thanks,
Mark

[Len 911]

Make a smoke machine and test for leaks.. I did this and noticed a leak between the cylinder heads. YouTube how to make a smoke leak tester... 5 gal bucket , solder iron, baby oil, air lines with a regulator... NO JOKE it worked pretty good.

[Dubbin']

Originally Posted by gtc

I've heard John Walker say that the vacuum line for the brake booster is often deteriorated and leaking by this point.
You issue sounds like something else, but it might be something for you to check.

Thanks I'll look into this. At this point I need to look at ALL vacuum sources...

Thanks,
Mark

[Dubbin']

Originally Posted by Len 911

Make a smoke machine and test for leaks.. I did this and noticed a leak between the cylinder heads. YouTube how to make a smoke leak tester... 5 gal bucket , solder iron, baby oil, air lines with a regulator... NO JOKE it worked pretty good.

Very good idea. I was sent a link by another member about a small smoke stick. It's only 50$, might not even be worth trying to Jerry rig one and it might be a good tool to find my leak(s)....

Thanks,
Mark

[wrxnofx]

On my '87 when I remove the oil cap while the engine is running (cold or warm) I notice no difference in idle at all. Like none. I don't have the stock air box on the car, however. Rather, it's one of the cone type filters. I don't know if this is the "problem" with having no dropout or not. I've replaced CHT, speed and reference sensors, O2 sensor, and the large hose from the tank. None of these made a difference. My tech removed my intake manifold last year to replace fuel lines, and those gaskets were replaced so I know there is no leak there, either.

My car runs fine and has no hesitation. The only time I EVER had any hesitation was when the idle control valve went south. I cleaned it a few times and it helped very temporarily. Idle was rough, and I'd notice major drop outs under acceleration. Replacing the ICV fixed this. However, my idle was rough and you said yours isn't.

The jumper test they're talking about is outlined here (which is taken from the Steve Wong instructions after changing to his chip):


I remember reading that you can test your brake booster by pumping up the brake pedal with engine turned off. Put your foot on the brake and start the car. If the pedal sinks, then the booster is properly pulling vacuum to pull the pedal down. If not, the booster is suspect or its vacuum hose.

[Tippy]

Originally Posted by Dubbin'

Yea I figure it can't be that bad... Ultimetly the airbox, afm and tb should be removed with the fuel rails, injectors and the blower... I don't really feel like taking all of that apart again but I know I might not have the choice... 2nd time around is always quicker I guess .

Thanks,
Mark

Definitely check the brake booster and the other gazillion vacuum lines before removing the intake.

Back to the intake removal, you don't need to pull the fuel rails off nor the TB. What I can't remember is if you need to pull the fuel lines off the FPR and return. You many not have to.

You're only lifting about an inch to clear the intake to head studs, so a lot of stuff and be left alone.

[Dubbin']

Originally Posted by wrxnofx

On my '87 when I remove the oil cap while the engine is running (cold or warm) I notice no difference in idle at all. Like none. I don't have the stock air box on the car, however. Rather, it's one of the cone type filters. I don't know if this is the "problem" with having no dropout or not. I've replaced CHT, speed and reference sensors, O2 sensor, and the large hose from the tank. None of these made a difference. My tech removed my intake manifold last year to replace fuel lines, and those gaskets were replaced so I know there is no leak there, either.

My car runs fine and has no hesitation. The only time I EVER had any hesitation was when the idle control valve went south. I cleaned it a few times and it helped very temporarily. Idle was rough, and I'd notice major drop outs under acceleration. Replacing the ICV fixed this. However, my idle was rough and you said yours isn't.

The jumper test they're talking about is outlined here (which is taken from the Steve Wong instructions after changing to his chip):


I remember reading that you can test your brake booster by pumping up the brake pedal with engine turned off. Put your foot on the brake and start the car. If the pedal sinks, then the booster is properly pulling vacuum to pull the pedal down. If not, the booster is suspect or its vacuum hose.

Awesome, thanks for the info. One question... What do I do with the 'jumper'? Do I need to plug something into it? Sorry if this is a dumb question... Are we relying on the tachometer to adjust the idle? Mine a a bit high so it might be good to bring it down...

Thanks,
Mark

[Dubbin']

Originally Posted by Tippy

Definitely check the brake booster and the other gazillion vacuum lines before removing the intake.

Back to the intake removal, you don't need to pull the fuel rails off nor the TB. What I can't remember is if you need to pull the fuel lines off the FPR and return. You many not have to.

You're only lifting about an inch to clear the intake to head studs, so a lot of stuff and be left alone.

You’re right. Now that I think of it, if I disconnect the fuel rails from the fuel lines/FPR I can probably pull the intake manifold up. It will make it easier if the AFR and airbox are off that’s for sure… Might as well plan on doing the gaskets... Worst case it's not bad maintenance...

Thanks,
Mark

[aston@ultrasw.c]

I had an intermittent hesitation under load on my 86 3.2.

I replaced all same crap and more

It ended up being the fuel injection wiring harness!

The factory folded the harness back on itself and stuffed it into a clamp, over the years the insulation broke down.

At idle, reach deep into the engine bay and pull/push the harness around, if the engine stumbles - you found it!

BTW the most conclusive vacuum leak test was a smoke test. I tried them all.

Good luck

[dersledite]

Originally Posted by Dubbin'

You’re right. Now that I think of it, if I disconnect the fuel rails from the fuel lines/FPR I can probably pull the intake manifold up. It will make it easier if the AFR and airbox are off that’s for sure… Might as well plan on doing the gaskets... Worst case it's not bad maintenance...

Thanks,
Mark

You've got it - I was able to do it with some mild frustration along the way in my old condo garage. I found one small crack in one spacer but it didn't turn out to be the source of the problem that I was chasing. I recommend you pressurize the vacuum system or use a smoke machine as suggested to determine you have a leak before you go this route... I pressurized the intake system to about 8-10psi using one of those tire pumps that plugs into the cigarette lighter. If the pressure doesn't hold, then start spraying various components and lines with soapy water to find the leaks leaks.

Also, even though it sounds like your AFM is in great shape, if you have it open again I would use a 9V battery to allow you to measure the resistance as the wiper moves across the track. This will rule out any flats spots.

Good luck

[Dubbin']

Originally Posted by aston@ultrasw.c

I had an intermittent hesitation under load on my 86 3.2.

I replaced all same crap and more

It ended up being the fuel injection wiring harness!

The factory folded the harness back on itself and stuffed it into a clamp, over the years the insulation broke down.

At idle, reach deep into the engine bay and pull/push the harness around, if the engine stumbles - you found it!

BTW the most conclusive vacuum leak test was a smoke test. I tried them all.

Good luck

Got it Thanks!

[Dubbin']

Originally Posted by dersledite

You've got it - I was able to do it with some mild frustration along the way in my old condo garage. I found one small crack in one spacer but it didn't turn out to be the source of the problem that I was chasing. I recommend you pressurize the vacuum system or use a smoke machine as suggested to determine you have a leak before you go this route... I pressurized the intake system to about 8-10psi using one of those tire pumps that plugs into the cigarette lighter. If the pressure doesn't hold, then start spraying various components and lines with soapy water to find the leaks leaks.

Also, even though it sounds like your AFM is in great shape, if you have it open again I would use a 9V battery to allow you to measure the resistance as the wiper moves across the track. This will rule out any flats spots.

Good luck

I figured the AFM was not the culprit since the hesitation also happens at WOT... The AFM, from what I’ve read controls mid/partial throttle and the WOT switch is the one controlling AF at WOT. I had cleaned it and reset the track because I figured it couldn't be harmful.

How are you pressurizing the vacuum system? I really want to know more about this… Where and how are you plugging the pump into the system?

Thanks,
Mark

[wrxnofx]

Originally Posted by Dubbin'

Awesome, thanks for the info. One question... What do I do with the 'jumper'? Do I need to plug something into it? Sorry if this is a dumb question... Are we relying on the tachometer to adjust the idle? Mine a a bit high so it might be good to bring it down...

Thanks,
Mark

No dumb questions here. We're all learning.

Use a paper clip and bend it so that you can stick it into the 2 green holes in the picture (this is the socket on the left side of the engine compartment behind the fuse cover, which you must remove). The paper clip acts as your jumper (you could also use a wire). When you stick the paper clip into the two green holes, the DME bypasses the ICV and no longer sends signals to it to modulate. As a result, the idle will be rock steady. While in this "jumpered" mode, set the idle with the screw shown in the image. Use a 1/4" socket and I think it's a 6mm or 7mm socket. Use the tachometer do view the engine speed. Given that the notches on the tach at this location are 250 rpm increments, you're getting it close. I suppose if you had an oscilloscope you could hook it up to read rpm that way and get more accurate. I don't have an o-scope.

[dersledite]

Originally Posted by Dubbin'

I figured the AFM was not the culprit since the hesitation also happens at WOT... The AFM, from what I’ve read controls mid/partial throttle and the WOT switch is the one controlling AF at WOT. I had cleaned it and reset the track because I figured it couldn't be harmful.

How are you pressurizing the vacuum system? I really want to know more about this… Where and how are you plugging the pump into the system?

Thanks,
Mark

I missed the note about WOT, you're probably right. You never know with the AFM though, I chased a rich condition at WOT for a long time and it ended up being the AFM after all.

Check post 13 in this thread, it a has a few pics of the rig I used to find a couple vac leaks. http://forums.pelicanparts.com/porsche-911-technical-forum/757777-help-educate-me-little-maf-upgrades.html

[Dubbin']

Originally Posted by dersledite

I missed the note about WOT, you're probably right. You never know with the AFM though, I chased a rich condition at WOT for a long time and it ended up being the AFM after all.

Check post 13 in this thread, it a has a few pics of the rig I used to find a couple vac leaks. http://forums.pelicanparts.com/porsche-911-technical-forum/757777-help-educate-me-little-maf-upgrades.html

Thanks, I guess I'll have to start fabricating something (I am in Montreal. If you don't mind shipping your contraption It would really be helpful... I would compensate you for the shipping...) . Is 8-10PSI the pressure that runs through those lines when the engine is running?

I'll read up about the 9V battery tests with the AFM. I will most probably be removing the AFM to do a lot of the other stuff so I can test it at the same time. I know I saw a thread/post about the AFM test.

Thanks,
Mark

[Dubbin']

Originally Posted by wrxnofx

No dumb questions here. We're all learning.

Use a paper clip and bend it so that you can stick it into the 2 green holes in the picture (this is the socket on the left side of the engine compartment behind the fuse cover, which you must remove). The paper clip acts as your jumper (you could also use a wire). When you stick the paper clip into the two green holes, the DME bypasses the ICV and no longer sends signals to it to modulate. As a result, the idle will be rock steady. While in this "jumpered" mode, set the idle with the screw shown in the image. Use a 1/4" socket and I think it's a 6mm or 7mm socket. Use the tachometer do view the engine speed. Given that the notches on the tach at this location are 250 rpm increments, you're getting it close. I suppose if you had an oscilloscope you could hook it up to read rpm that way and get more accurate. I don't have an o-scope.

Got it. Thanks. I'll try adjusting the idle also. Ultimetly I figure there would not be a benefit to my hesitation although I guess it is a good thing to do while trying to get it running to factory spec.

Thanks,
Mark

[TT Oversteer]

When you sprayed carb cleaner around the intake manifold-to-head gaskets, did you also check the rubber seal that joins the two halves of the intake manifold in the middle just below the throttle body? These can leak also as I learned with mine. Easy to replace without completely removing the intake.

[eolson]

I think your comment about it happening at night points to something to do with mixture. It made me think of air density and such, and that maybe something is compensating (or not) when the air is cooler. Sorry I can't be more specific, that part just jumped out at me.


Sent from my iPhone using Tapatalk

[mnez]

Originally Posted by aston@ultrasw.c

I had an intermittent hesitation under load on my 86 3.2.

I replaced all same crap and more

It ended up being the fuel injection wiring harness!

The factory folded the harness back on itself and stuffed it into a clamp, over the years the insulation broke down.

At idle, reach deep into the engine bay and pull/push the harness around, if the engine stumbles - you found it!

BTW the most conclusive vacuum leak test was a smoke test. I tried them all.

Good luck

I also had an intermittent misfire under WOT that felt a bit like a rev limiter.
After going through all the same stuff (and lots more), it turned out to be a bad connection at one of the pins on the 4 pin connector for the fuel injector harness, located on the firewall. Only found after opening the connectors and finding one pin loose in the socket. Cleaned and spread the pins, problem was solved. Now the connector has zip ties to keep it tight together as well.

So don't discount an electrical problem...
Only found it when

[steely]

Here is more info on the aforementioned brake booster line - Jump to the TEST in this one.
http://forums.pelicanparts.com/porsche-911-technical-forum/435952-vacuum-leak-you-probably-didnt-ever-think.html

The air valve / vac limiter fitting may need attention and is a cheap fix from Autozone for a PCV baffle for a couple of bucks – the Baffle AZ#5333.
http://forums.pelicanparts.com/porsche-911-technical-forum/671417-oil-breather-line-id-86-carrera-3-2-a.html

The pipeline (part name reference from the PET) behind the TB can be a problem – check for cracks - don’t buy the expensive part – get 2 different diam vac rated lines, fit one length inside the larger and clamp it down – see my picture where I wrote “fixed”.

Get a small qty of vac lines from your FLAPs and replace the lines.
Check the intake rubber that TToversteer mentioned earlier.
Connect all of the lines to the charcoal canister, TB, Air Box, cruise, etc. See vac diagram.

You mentioned that your idle is fine so your idle switch must be too. I would recommend you still check it since the switch could be stuck in the idle position (signal to DME), while your engine isn’t idling.



The ? in the diagram is a cloth braid line vented to ambient air in the vicinity of the firewall.


HTH