Early 944 Temp Sensor Issues

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delgadostarside
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Hey all I'm new here at carpokes. I've been trying to diagnose a weird sensor issue i've had on my 83' 944 project. A little background about the car, I purchased it about 13 months ago where the car was in rough shape, missing the entire interior and a rusted out floorpan. I've been meticulously restoring the car from the ground up between welding in new sheetmetal for the pans and coating it all in KBS to ripping out the entire drivetrain and refreshing everything with new parts. I'm yet to even drive her but I'm at the end of the mechanical side of things and have started on the interior while trying to diagnose this last step. Also i've rebuilt the engine in my backyard replacing the rod bearings and all gaskets and seals.

I've started a post over on Rennlist but I'm hitting a brick wall with what else could be causing the problem. The problem is that the engine presents a no start until the DME temp sensor is unplugged, then the car immediately starts. I've also been chasing down a leak down problem where the rail depressurizes immediately after engine stop. The engine also runs pretty rich and cylinders 1 & 4 have had some heavy soot after running since the rebuild. In terms of diagnsotic steps this is what I've done so far:

-Checked DME temp sensor resistance with Clarks Garage (Temp sensor passes and is brand new from the rebuild)
-Checked AFM for deadspots and voltage matching Clarks Garage
-Replaced TPS as the one I had previously did not pass the resistance check from Clark's Garage. Adjusted so the click happens once the throttle begins to open and clicks off when closed.
-Replaced FPR as the fuel pressure didn't pass for idling or leak down test. After replacing the fuel pressure has been the same, never reaching over ~30psi
-Checked for injector leaks by removing the rail, putting cups under each injector and pressurizing the rail by jumping the pump. No leaks around seals or coming out of the injectors.
-Checked for Dampener leaks removing the vacuum line and pressurizing the rail the same way.
-Checked ICV for proper operation. I put a 12V power supply and saw the valve open slowly.

In terms of the pressure leak down im down to thinking it's the check valve at the fuel pump but the running rich & no start with the temp sensor plugged in really doesn't make sense. I was wondering if anyone has any ideas about diagnosing this beyond what I've done so far. The original post is here as well in case I missed anything:
https://rennlist.com/forums/924-931-944 ... oodoo.html
I appreciate any and all help! :)

#1

944er
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Your low fuel pressure results in too lean a mixture. Unplugging the dme temperature sensor tells the dme it is extremely cold, so it tries to make the mixture very rich. Combined with the low pressure this results in a combustible mixture.

I suggest fixing the fuel pressure issue first. Then run an injector balance test to see why 1 and 4 are different than 2 and 3.

#2

delgadostarside
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That’s actually a great idea. I have a homemade injector tester i made from an Arduino and relay board but i can definitely write a short program to make a balancer out of it. I’m going to replace that check valve first and go from there. That makes a lot of sense why it’d be running rich aswell. Thankyou!

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usury
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While you're working near the fuel pump for the check valve, you'll probably find some pretty crusty 40-year-old fuel lines.

I wrote this post about Replacing Tank-Side Fuel Lines on my late (944.2) car. Don't know how different the early fuel pump area might be, but that post is a starting point for what you might find
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1987 944na with S2/Turbo facelift
Seattle, Washington, USA

#4

sourcreamfries
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Definitely check those old fuel lines while you're in there.

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delgadostarside
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Hey all, so the check valve came in today. I'm gonna hold off on the fuel lines for now and come back to them later although i will definitely change them, those fittings were real rusty. However, replacing the valve i did another leak-down test. After 20 minutes she came down from 31 to 18 psig! Finally she holds pressure! On another note, I attempted to start her after this to see if she'd start up with the sensor plugged in, and after about 10 seconds or cranking she started up! no funny business needing to pull the sensor!

So that aside, I went to do a balance test I end up getting just about the exact same results across the injectors...
Balancer Test.JPG
Balancer Test.JPG (30.66 KiB) Viewed 2838 times
So from here i can assume cylinders 1 & 4 being dirty are attributed to something else other than the injectors.

For anyone curious, the balancer i made uses an Arduino to trigger a relay to stay on for 40ms then turn off for 40ms and loops this 10 times. I found that 20 times would completely depressurize the rail on all injectors so i stuck with 10. I might mess around and lower the off time between pulses or the amount of time the injector stays open to see if I get any different results with the test.

So success? She starts and the injectors are assumingly working as intended. Is it just everything breaking in for why 1 & 4 are rough or did i make the BIGGEST mistake of not changing the rings on the rebuild? Cant see why cylinder 2 wouldn't be dirty if it were the old rings were causing the low compression though...

#6

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Tom
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Sounds like some success there. I'm unclear where things stand? Is the only remaining question about why cylinders 1 and 4 seem to be sooty? Where are you seeing the soot? On the plug? Black soot usually means it's rich, but could also mean those cylinders are missing. Since you've had a lot of no-starts, it might also just be a result of that. I'd clean them up or change them and see if it keeps happening on those cylinders only now that the motor is running smoothly.

How are you measuring fuel pressure on a cylinder by cylinder basis, and also measuring flow quantities? Or am I misunderstanding your chart?

#7

delgadostarside
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Hey so i wasn't exactly clear about what i did and about the dirtiness since that was on Rennlist. Before making this post, i had pulled the plugs and noticed the plugs for cylinders 1 & 4 were very sooty. I also pushed my bore-scope in all the cylinders and confirmed that 1 & 4 were dirty on the pistons heads and valve areas while cylinders 2 & 3 were practically sparkling clean. After seeing this, I ran a compression test on each cylinder and had:
Cyl 1: 125
Cyl 2: 140
Cyl 3: 145
Cyl 4: 125
This is where i thought my head studs maybe hadn't seated evenly during the torque down process so i re-torqued them according to Clark's waiting 15 minutes between stages. I should mention the plugs are brand new and were put in during the rebuild. The car had also only been ran a total of 20 minutes since the rebuild. After seeing they were sooty though, i cleaned them with some brake clean and rechecked the gap before putting them back in. From this, i started this post here on Carpokes.

About the fuel pressure measurement, i have a fuel pressure gauge on the end of my rail where i pressurize it by running the pump for about 3 seconds till it reaches its maximum on the gauge (about 31psi) and record it. Then I start the tester, once it finishes pulsing i record the pressure gauge reading and move onto the next injector. I put a dixie cup under each injector so i wasn't dumping it all into my cylinders but i wasn't actually measuring the flow in terms of volume. I can assume the volume flow rate is proportional to the pressure drop before and after the injectors pulse and balance them that way basically. I havent taken another compression test since re-torquing the head but I should probably see what results i get now. I am hesitant to remove the head just to clean the cylinders since i'll have to flush the oil and coolant so if anything ill have to get a small enough brush to fit in the plug holes.

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delgadostarside
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So did another compression test today across the board i get:
Cylinder 1: 120 psig
Cylinder 2: 135 psig
Cylinder 3: 145 psig
Cylinder 4: 145 psig
Im guessing everything is wearing in? or the head is warped? Then again, cylinder 1 already had an issue from P/O. Another thing. I couldn't get her to start today with the sensor plugged in at first. Im sure my Air/Fuel is off from that bleed screw, i did mess with it when i first got the car along with the idle. Stupid mistake. I messed with the idle and bleed screw again till i got her to start up and idle appropriately. I'm going to get my oscilloscope on her tomorrow and try to adjust the idle properly while adjusting the bleed screw. Right now, the bleed screw is about halfway turned up the hole. I'll try to find the procedure too for it now that everything else is working properly.

#9

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usury
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This video about idle adjustment came through my YouTube feed a couple days ago. Perhaps it will be useful to you...

Freelance New Age Renaissance Man
and Wrench Warrior
1987 944na with S2/Turbo facelift
Seattle, Washington, USA

#10

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