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1985.5 Porsche 944 Engine Hesitation

Old 11-02-2018, 11:49 PM
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pointFive
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Default 1985.5 Porsche 944 Engine Hesitation

After replacing several components on the car (list can be found in the engine hesitation clip), the idling has changed dramatically. There's been a noticeable difference in acceleration (poor) and an apparent odor of gas/fumes...

The engine idles between the the 1000 rpm mark and the smaller mark below it, so ~900 rpm. I did have to adjust the idle speed after installing the throttle response cam, which had lowered the idle below 800 rpm...

I checked to insure all vacuum lines/hose were intact, but haven't looked over anything else.

What are common areas to inspect regarding this issue? Are there any tests that require a DMM? I searched for similar threads that lead me to believe it could be anything from bad injectors to an out of spec TPS and/or temp. sensor...

Components that have been replaced/tested (uddated 1/6/2019):
  1. Spark plugs (gapped at ~0.029")
  2. Spark plug wires
  3. Distributor cap
  4. Distributor rotor
  5. Air filter
  6. Throttle response cam
  7. Timing belt
  8. Balance shaft belts
  9. Water pump
  10. Fuel filter
  11. Thermostat
  12. DME temperature sensor
  13. Fuel pressure test (results below)
  14. Noid lite test
  15. Engine motor mounts
  16. Intermediate steering shaft
  17. Brake booster
  18. Brake master cylinder
Here are two clips of engine during a cold start: Engine Hesitation -
Exhaust Note During Hesitation -

Last edited by pointFive; 01-06-2019 at 02:35 PM. Reason: Updated running list
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Old 11-04-2018, 05:26 PM
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I spent some time on the 944 this morning hoping to pinpoint the issue...I checked the gap of the spark plugs and re-gapped them to 0.029". I did notice the terminal threaded tops were loose so I carefully tightened them with some pliers. The plugs I installed are Bosch WR8DS and the wires were replace as well(Beru)...

I also cleaned the throttle body; which had a good amount of oil buildup, the throttle position sensor, and replaced the fuel filter, yet, there is still a hesitation at idle and "popping" from the exhaust... Attached is a picture of an old plug (all having the same wear) and the throttle body before I cleaned it...maybe they'll help diagnosis the issue.

Is there a vacuum line diagram available for the 944 8v? Maybe there's a hose/line that I was unable to catch...and speaking of vacuum lines/hoses, what's their purpose? I'm hoping the injectors haven't failed because they are quite expensive..Lastly, are there detrimental effects to the engine if I drive the car while I'm self-diagnosing it? At the moment it's my daily driver...


Last edited by pointFive; 11-04-2018 at 08:58 PM.
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Old 11-04-2018, 10:01 PM
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Old 11-06-2018, 02:32 PM
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I just placed an order that included a compression tester, noid light tester, fuel pressure tester, and a DME temperature sensor...

On my way to work this morning, the oil pressure flashed several times during my commute. I had recently changed the oil (Valvoline VR1) and checked it's level this past Sunday...

I can't confirm if engine vibration caused the oil pressure sender to trip...the motor mounts are shot so there is above normal vibration...

Is there anything else I can check that may be causing this hesitation/popping, as I wait for the parts to come in?
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Old 11-06-2018, 09:30 PM
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A new DME temp sensor is a sensible first step. Good to rule it out. I would say that most idle problems like you describe could be attributed to a vacuum leak. This upsets the mixture and makes the car run lean. Unfortunately there are a lot of hoses that can be old and cracked.Most of them are under the manifold. At one point, trying to get a car to idle, I took off the manifold and plugged everything on the manifold and then gradually worked my way out until I found the problem. One standard trick is to spray starting fluid (ether) at various points along the hoses and see if the engine speeds up. If there is a vacuum leak, it will suck in the ether and momentarily make the engine run faster.

When you get your fuel pressure gauge, go to Clarks-Garage and run the fuel pressure checks he recommends. The fuel pressure regulator does drop the fuel pressure during idle, when operating correctly. He describes the numbers you are looking for.

Noid light will be helpful. Make sure all 4 spark plugs are firing. Also put your finger on each fuel injector and make sure you feel it clicking.

Keep on doing diagnostics. You'll find it!!!
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Old 11-07-2018, 12:11 PM
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harveyf Thanks for the momentum! The vacuum hoses/lines seem to be original. Does anyone sell a vacuum hose/line kit? Can silicone hoses replace the thin plastic lines? If I were to take of the intake manifold, would I need to install a new gasket?

Also, there was a good amount of oil build-up in the j boot and throttle body when I had taken it apart to be cleaned...do you know where I can purchase AOS seals?
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Old 11-07-2018, 02:33 PM
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Vacuum line kits are everywhere, mostly silicone yes. You can also get bulk and cut yourself. Sorry, I forget the size... 3.5mm? If you remove the intake, prepare 4 intake gaskets in advance and put the front of car one on backwards. AOS seals very available. I got mine from Porsche, but any of our P-car suppliers should have them.

Search Porsche 944 PET for parts diagrams and numbers
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Old 11-07-2018, 02:55 PM
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Dan Martinic I appreciate your input! Regarding the position of the "front of car one" gasket, why do you install it backwards?
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Old 11-07-2018, 02:59 PM
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Originally Posted by pointFive View Post
Dan Martinic I appreciate your input! Regarding the position of the "front of car one" gasket, why do you install it backwards?
More like upside down; it's obvious if you pay attention (the hole in the head is orientated the other way from the rest)

Google Laust vacuum manifold... very cool way to replace vac lines!

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Old 11-07-2018, 03:10 PM
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Ahh, thanks for the clarity...

The Laust vacuum manifold is very cool indeed...
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Old 11-08-2018, 10:28 PM
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Point Five, don't just concentrate on the small vacuum lines. Everything downstream of the throttle body is under a vacuum. This unfortunately includes a bunch of thumb sized lines under the manifold that are expensive to replace and thus may be old and cracked. Try the starting fluid first to see if you can spot a specific place that is leaking. And any leakage downstream of the air flow meter will disrupt the fuel/air mixture and give poor idle performance. This includes leaks in the "J boot".
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Old 11-08-2018, 11:08 PM
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Good point Harvey! I once finished a big job and forgot to plug in the Vac line from AOS to J-boot. Car didn't run; took me a bit and a reminder from Tom to find that lol

Say, what is this mysterious noid light you speak of? The standard kits don't seem to have any that work: the Bosch one doesn't fit and the GM one that does won't light. I use a cheapo 12v LED. Works great! Got that tip from fast951
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Old 11-09-2018, 10:26 AM
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Personally, I use my timing light to establish that each spark plug lead is firing. I don't actually own a noid light.
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Old 11-09-2018, 11:46 AM
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Originally Posted by harveyf View Post
Personally, I use my timing light to establish that each spark plug lead is firing. I don't actually own a noid light.
Interesting. How exactly do you do that?
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Old 11-09-2018, 11:47 AM
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Dan Martinic I purchased this
noid light noid light

I haven't used it yet...

I'll let you know if the the Bosch lit cover works; finger's crossed...
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