BrewCity20
BrewCity20 New Reader
7/23/19 3:16 p.m.

I've been working on a high millage 1986 Porsche 944 NA (it sat for about a year before I got my hands on it) with the hopes of turning it into a workable track car (and even hopefully use it yet this summer). With that said, I have since replaced the brake and fuel lines and replaced the oil. I am now able to start driving the car around town to get a hang for it / shake down cruise. With that said, I've noticed that the engine doesn't seem to run very well when cold.

More specifically, when driving with the engine cold I have had a few instances where the car would just not produce any power - it bogged down to the point where it took everything I could to try to keep it going, much less accelerate. I actually had to pull over so a car behind me could pass because I couldn't get up to the 40 mph speed limit. Strange thing is, once the engine warms up a bit this goes away and it runs relatively smooth without any issues.

Any ideas/suggestions on what may be the problem? I am a bit nervous bringing a car with this type of issue to the track and risk it bogging down during a session.

My plan is to continue to take it on trips to try to break it back in since it sat so long before I purchased it - continue to get heat in the engine and cycle the brakes. I am also hoping to do a coolant flush in the next week or so (not that I expect that to address the issue but just to freshen things up).

Any help is greatly appreciated.

Stefan
Stefan GRM+ Memberand MegaDork
7/23/19 3:22 p.m.

Could be the engine temp sensor (or its wiring) or the Air Flow Meter (or the intake J-boot) or even the idle speed valve.

Clarks-Garage has some great write-ups that might help:

http://www.clarks-garage.com/shop-manual/ts-01.htm

BrewCity20
BrewCity20 New Reader
7/23/19 3:27 p.m.

In reply to Stefan :

This may be the easy answer to my question then. I had to take the air filter housing assembly off the intake to gain access to the brake elements (we are also planning on doing an AC delete bracket) so I ran that test drive without it attached. 

Would putting that back on possibly clear it up?

irish44j
irish44j MegaDork
7/23/19 3:28 p.m.

^^ just listen to whatever he says. Whatever he says is what I would probably say as well lol. 

But, sounds like engine temp sensor. 

Stefan
Stefan GRM+ Memberand MegaDork
7/23/19 3:41 p.m.

In reply to BrewCity20 :

The air box is fine, but the AFM and the J-boot are very sensitive to extra air. 

I'd start there and make sure there aren't any extra leaks.

BrewCity20
BrewCity20 New Reader
7/23/19 3:50 p.m.

In reply to Stefan :

Thanks. I will take a look at the AFM, J-boot and engine temp sensor and see if anything turns up.

Much appreciated.

pilotbraden
pilotbraden UltraDork
7/23/19 7:03 p.m.

A small leak between the airbox [filter housing] and the intake manifold renders my 944 inoperative.

Knurled.
Knurled. GRM+ Memberand MegaDork
7/23/19 8:06 p.m.

Old-school Bosch K- or L-jetronic is very intolerant of false air.

 

There must always be an impermeable path between the airflow meter (or fuel distributor) and the intake valves.  99% of the problems people have is because they figure "eh, good enough".  It's sort of like that one movie with the Russian airplane and Clint Eastwood.  You must think in German.  There is no good enough, only precision.  You must have exactly zero false air.

 

Footnote:  I'm probably the only person who preferred having a Digifant controlled Golf to a CIS Golf.  Digifant worked BETTER, but it was a hell of a lot less fault tolerant.

Stefan
Stefan GRM+ Memberand MegaDork
7/23/19 8:21 p.m.

In reply to Knurled. :

The 944 uses a Motronic EFI system, similar to the BMW.

The 924 NA and Turbo with the 2.0L uses Bosch CIS and it’s why so many are available cheap.  The fuel systems are a mess after sitting due to the terrible grounds and ham-fisted mechanics “fixing” them.  Plus the 2.0L is a gutless wonder in the US and fixing that isn’t as easy as other VW/Audi engines of the time.

Knurled.
Knurled. GRM+ Memberand MegaDork
7/23/19 8:27 p.m.

In reply to Stefan :

The 944's system is almost identical to a mishmash between Digifant and the K-jet based Motronic used on turbo Audis.

 

"American, Russan, all made in Taiwan!".   It's all the same Bosch pre-OBD-II E36 M3.  Not fault tolerant much.

It reads crank signals like an Audi 5000CS, it controls fuel and spark like a Digifant II controlled Golf. 

 

 

Digifant II is just L-jet with ignition control.  Nothing fancy at all.

 

(No, I am not getting my Ks and Ls mixed up.  Audis with turbos all had K-jetronic, but controlled with a computer that read a "tritach" signal the same way 944s did, with the one crank sensor that read flywheel teeth and the other that read a TDC Incoming pin)

jr10cross
jr10cross Reader
7/23/19 8:49 p.m.

In reply to BrewCity20 :

I'd really check the j boot to make sure there are no holes and that you've fastened it back tight over the throttle body lip. It sounds like you have way too much air going into the throttle body when the engine is cold. The additional air messes up the fuel to air ratio. The ECU will usually increase the fuel and decrease the air flap during cold condition, but if you have a hole or gap it will throw everything off. Once the engine is warm the addition air doesn't affect performance as much.

 

Check out my YouTube channel, I got tons of 944 stuff. My cars were pretty cheap and I turned them into daily's. 

 

irish44j
irish44j MegaDork
7/23/19 10:20 p.m.

in particular, check the "nipple' for the hose on the bottom of the j-boot. When you look at it on the car, it can look fine but actually be cracked most of the way off.....I know this from experience :)

BrewCity20
BrewCity20 New Reader
7/24/19 2:06 p.m.

Thanks for all the input, I will take a look at the J-boot and see what I find. The previous owner had clearly messed with it in the past because it has brand new hose clamps on it. That may be the issue.

Will update once I have had a chance to try it.

BrewCity20
BrewCity20 New Reader
8/5/19 8:27 a.m.

So to update on this. I went and looked J-boot and it appears there were some loose hose clamps on the two smaller "secondary" openings. The previous owner had clearly replaced the boot more recently as the rubber looked brand new. I tightened those down and the engine has been running smoother.

I still get a minor hicup where the engine doesn't want to make power when cold but the effect is greatly reduced (e.g., the engine bogs for about 30 seconds of running the first or second time I really try to give it power) and then the engine is good for the rest of the day without issue. 

I plan on running some 44k fuel system cleaner through the tank this week in anticipation of our first autocross day on Sunday. I suspect the Injectors may be a bit gummed up given how long this thing sat before we fixed it up. Hopefully that puts us in position to at least make a run of things on Sunday.

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