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1985.5 Porsche 944 Engine Hesitation

Old 11-09-2018, 11:54 AM
  #16  
Dan Martinic
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The kit I had and returned was also OTC but only had one Bosch noid light; it didn't fit. Yes, let me know if one of yours does and which one. FYI one of the GM ones fits but doesn't light
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Old 11-09-2018, 11:57 AM
  #17  
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What is this, hesitation season? On my way to work this morning, I barely made the final two miles: got this sudden hesitation when applying throttle! Comes down to idle just fine. Only the gentlest of throttle application coaxes it to go properly.

Reminds me of a time one winter when the boost kept cutting out; I think it was bad fuel (knock sensor). Geez I wonder how it will run at the end of this day
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Old 11-10-2018, 03:00 PM
  #18  
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Dan, I just move the inductive pickup around from 1 to 2 to 3 to 4 and look for the light to fire.
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Old 11-10-2018, 03:08 PM
  #19  
Dan Martinic
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Originally Posted by harveyf View Post
Dan, I just move the inductive pickup around from 1 to 2 to 3 to 4 and look for the light to fire.
Got it. So obvious I missed it lol

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Old 11-10-2018, 09:43 PM
  #20  
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With hesitation I think you have to consider the AFM. The variable resistance wiper track wears and misreports the amount of air coming in so he DME doesn't know to inject more fuel. Engine will kind of behave like there is a big vacuum leak - a lot of unmetered air coming in.
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Old 11-10-2018, 09:54 PM
  #21  
Dan Martinic
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Originally Posted by gtroth View Post
With hesitation I think you have to consider the AFM. The variable resistance wiper track wears and misreports the amount of air coming in so he DME doesn't know to inject more fuel. Engine will kind of behave like there is a big vacuum leak - a lot of unmetered air coming in.
Hmm.. A couple of weeks ago, I opened the AFM up and did the whole "clean-up & bend the contact" thing. Seemed to respond a little better and my cold morning-start 30 sec hesitation disappeared! But this new hesitation is different. Yesterday, it totally died on me--then after a minute, fired right up. Drove a bit further, dead. No idle even. Waited... started up like nothing's wrong. Made it to the gas station... ran & died ran & died ran & died some more. Finally, ran for a long time so I drove on the rest of the evening. Gonna make a new post about this; I'm thinking fuel pump or ignition switch. See new thread!
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Old 11-12-2018, 07:54 PM
  #22  
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Dan Martinic The size and shape of the connectors on the Bosch 2 (picture attached) worked perfect. This noid lite cover came from my OTC 3054e kit...

All the injectors were receiving a fuel pressure pulse signal and I could feel the internal clicking, so I think it's safe to rule out a clogged/poor injector...

gtroth I plan on servicing the AFM. There's a hesitation as I accelerate and the AFM could certainly contribute to that...

This week, I plan on replacing the DME temp. sensor, then check the fuel pressure and the compression of each cylinder.


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Old 11-12-2018, 09:59 PM
  #23  
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Thanks for the picture and confirmation--good to know! (For Ontario readers: our commonly found kit -- OTC "red box" -- only comes with 1 Bosch noid light and it would appear it's the non-fitting Bosch 1)
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Old 11-18-2018, 09:37 PM
  #24  
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Update...

I replaced the DME temperature sensor and tested the fuel pressure; by following Clark's-Garage...

The 20 minute leakdown pressure was out of spec, which was well below 1 bar...could this contribute to the hesitation?


Relay jumpered

Engine running at idle

20 minute leakdown


What should I test or replace next?
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Old 11-19-2018, 10:40 PM
  #25  
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I've been experiencing something quite unusual...

Right after I turn off the car, it tries to start itself again...it doesn't happen every time, but it has happened more than once...

Would this odd phenomenon have anything to do with the hesitation/misfire?
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Old 11-19-2018, 11:47 PM
  #26  
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Originally Posted by pointFive View Post
I've been experiencing something quite unusual...

Right after I turn off the car, it tries to start itself again...it doesn't happen every time, but it has happened more than once...

Would this odd phenomenon have anything to do with the hesitation/misfire?
i suppose a flaky ignition switch could cause both engine cut-outs and restarts.
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Old 11-19-2018, 11:48 PM
  #27  
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It does what?? You turn the key to OFF and it activates the starter?
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Old 11-20-2018, 09:40 AM
  #28  
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I'm not going to conjecture on your restart issue without more info, But regarding the fuel pressure, the main downside of the fuel pressure dropping off is a slighter greater chance of vapor lock on hot days and of course a slight lag upon initial start as fuel pressure builds. Vapor lock- the fuel flashes to a vapor in the line near a heat source, mainly results in the car not starting. So I don't think the drop off in pressure with the engine off has anything to do with your hesitation. It just means the check valve back at the fuel pump is leaking by.

A test you should make is as follows: Get the car in a stable idle. Note the fuel pressure. It should be around 29 psi (per Clark). Remove the small vacuum line that runs to a tee/fuel pressure regulator/fuel damper. Remove the line at the throttle body connection. You should see an immediate jump in fuel pressure to around 36 psi (per Clark). This is the effect of the fuel pressure regulator at idle i.e. it pulls the fuel pressure down from max to 29. If it doesn't do this, there is a problem and the car is effectively idling at a rich mixture (higher pressure > more/too much fuel). This could cause rough running at idle. None of this would affect acceleration and full throttle performance, in a direct sense. Also, when you remove the line at the throttle body, you are introducing a vacuum leak. Put your finger over the nipple to teach your ear what a small vacuum leak does to idle quality. Not much, I suspect, but there should be a small change between finger on and finger off.
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Old 11-26-2018, 04:24 PM
  #29  
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Default 1988 944 NA

My 944 is lagging pretty drastically, I recently had it sputter out and sound horrible on me, has to get it towed home, where I later was told to check the camshaft bolt to see it was tight or loose.

Sure enough the thing was loose.

I tightned it to the torque specified, also changed the oil and the Oil Pressure Sending unit.... also found a random vacuum hose in the engine well and I do believe I put it in the correct spot. It was coming off of the vapor sensor..

so I drive it more. I have to go to work, I notice now itĺs accelerating at probably 50 percent. Maybe in 5th gear I can say it almost feels normal. It reminds me of when my J-boot had a crack, sorry for going on, but I noticed the other day a fuel injector leaking, closest to the radiator, so I took the fuel rail off and cleaned and checked the o-rings, also watched some YouTube videos and made a little fuel injector tester, but when tested each enjector did a bad job I would say. So I tested the resistance with an Ohm meter and it was way off, do I need new fuel injectors or can I try to rebuild these. Also what is the size for the Vacuum hose?! I would love to redo all of them.
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Old 11-26-2018, 05:35 PM
  #30  
Dan Martinic
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Vapour sensor? What's that?

If they are leaking from the body itself, I don't believe the injectors are rebuildable. For the smaller vacuum lines, check a vendor's website ex. Lindsay; I believe they are 3.5mm or imperial equivalent. The larger lines are tricky as some have odd bends and fasteners--best to buy exact replacements rather than bulk

Last edited by Dan Martinic; 01-06-2019 at 10:23 PM.
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