D-Jet problems, temp sensitivity issue Options

[mgphoto]

Hello and thank you for looking.

I have had a very stable car for the last 29 years, stock 1.7 with original D-Jet, except for the addition of a Per-Lux Ignitor (installed 20 years ago, that was the name back then).

The car is maintained and serviced by me and over the years I've replace leaky injectors, vacuum hoses and the AAR, nothing really major has gone wrong with the fuel system.

Little more than a week ago, Los Angeles got a burst of winter, temps in the mid 40's overnight and my very stable car began to have problems with warm up and running.

Starting does not seem a problem, as when cold (at ambient temperature) the engine will start and run for about 30 seconds, long enough for the AAR to begin to close and the idle to drop.

When this begins to happen, the engine begins to stall, the idle drops to 0 but bounces back up, this will happen several times until the engine stalls.


If I am lucky I can restart the engine and it will idle, (at this point if I am not so lucky I will have to restart the engine upwards of 15 to 20 times after which it will begin to idle).


At this point I can not touch the accelerator pedal, pressing the pedal will increase the RPM's but only momentarily as the engine will stall.

After the car has idled for about 10 minutes, it becomes somewhat drivable, sometimes with severe bucking, sometimes stalling.


Within 15 to 20 minutes the oil temp is starting to register on the gauge, and the car runs as it always has, very strong with any hesitation, totally repeatable every time.


I read a number of articles here on 914World about F I problems and the CHT seemed to be the culprit.

Simple enough, swap it out and see what happens.

Well I won't try the glue the washer to the sensor again, as soon as it came in contact with the head the glue broke and the washer fell into the fins on the head.

Just lucky to have another new sensor with another washer on hand.

Not to lose the last new washer I had, I decided to peen the inside of the washer with a small screwdriver and a hammer, in four opposing places, enough to cause the washer to crimp so it would catch the treads and screw onto the sensor so it could not fall off.

Replaced the CHT and found no change!

I decided to look at the temp aspect.

I have been monitoring the engine block temp, head temp and ambient temps with a laser pointer temp probe.

At this point, I believe the head temp of between 70 and 85 deg F is the point were the stalling occurs.

At head temps above 100 deg F the car will idle, at temps above 140 deg F the car will run.

Low ambient temps causes the AAR to stay open longer, the heads heat faster.

Higher ambient temps may cause the engine to stall faster.

Once the oil is warmed up to operating temp the engine runs all day without problems starting and restarting after several hours (engine temps still over 100 deg F)

I believe the problem is fuel related, if it were ignition there would be backfiring when the ignition kicked back in, fuel would still be delivered and pushed though the engine to ignite in the exhaust system.


Has anyone experienced this kind of temperature sensitivity and is there a cure?

Thanks in advance your help in the matter,
Mike

[TheCabinetmaker]

I think your aar is not working. Disconnect the hose from the aar to the throttle body and see if that helps. If it does, the aar is stuck closed. You can sometimes free them with some pb blaster.

[sean_v8_914]

I agree with VSG914.
the AAr is slow even in perfect condition it takes longer than 30 sec to close. pull the hose and check. is it open when cold? how long does it take to close? PB blast it, clean it. you can remove it and bench tets it easier. ground the body and connect the wire to positive and watch it close. I bet its stuck closed or mostly closed.
does your ECU have an idle mix adj?

[Cap'n Krusty]

Make sure the intake air temperature sensor is working and within specs.

The Cap'n

[sean_v8_914]

capt, have you seen those go bad? wrong resistance range or just open?

[mgphoto]

Make sure the intake air temperature sensor is working and within specs.

The Cap'n

Hey Cap,
I considered just replacing it, but not convinced it is the problem, I will test it.

Also if I pull the plug on the intake air temp sensor the car should just run rich, I'm told this is a trick for a little better performance and will sacrifice fuel economy and if I understand it's function it will change the mixture slightly to compensate for air density.
Thanks,
Mike

[mgphoto]

I agree with VSG914.
the AAr is slow even in perfect condition it takes longer than 30 sec to close. pull the hose and check. is it open when cold? how long does it take to close? PB blast it, clean it. you can remove it and bench tets it easier. ground the body and connect the wire to positive and watch it close. I bet its stuck closed or mostly closed.
does your ECU have an idle mix adj?

Hello Sean,
I believe the AAR is functioning correctly, even if it has failed pressing the accelerator should induce enough air to keep the engine running.

Simple test should be pulling the vacuum hose to the intake manifold that should keep the idle high as long as it is disconnected.

Yes the ECU has the idle mix adjustor.

Thanks,
Mike

[timofly]

Does your ECU have the mixture adjust knob? If so, try turning it clockwise about 4-6 clicks. That richens the mixture, I'm told.

I had the same problem and that fixed it. Colder air = denser air, so I'm guessing that the richer mixture was needed to maintain proper idle mix. I would have thought the ECU system would handle it, but fiddling with the ECU mixture knob solved the problem.

[r_towle]

For kicks, test your cht when cold and hot to verify it works.
The ambient air sensor may be out of spec.

Sounds like either the CHT or the MPS is dying.
I know you replaced the CHT, but maybe it was not the right part number??? check the resistance and match it to what your ECU needs to see...each CHT is different and must send a specific range to the ECU during warm up.

There is an idle circuit in the ECU that is driven by the position of the TPS and those do get worn out or dirty after 40 years...
TPS position, valve adjustment and timing are critical in cold weather.
You may have valves that are a bit to tight...that will cause erratic idle until they are warm..
To loose is better than too tight, especially the intakes...they cause really poor idle when cold.

FI Trigger points may also be a factor just purely based upon age.
mine were.

Here is my setup in the Northeast...and it gets darn cold here.
I drive year round...never an issue with warm up.
No AAR
No Cold Start Injector (wiring or fuel)
No Decel Valve

I have replaced everything else.
New trigger points (weird symptoms...last thing I changed, fixed all my issues)
new ignition every 6 months for me...points, cap, rotor, wires, plug
New CHT
New TPS
New injectors (as they failed)
New injector seals (twice now...long story)
New intake gaskets
All new vacuum lines.

I would suggest that you re-check your valve adjustment when cold.
Recheck your timing and dwell
Recheck your CHT for proper operation.

Rich

[mgphoto]

Hello again,
Temps today were in the mid 60's deg F, after sitting in the garage for a full day (rained yesterday) I measured the engine bay temp at 68 deg F.
I started the engine and did not have enough time to completely back the car out of the garage before the engine stalled (maybe 20 seconds). Very difficult to start, upwards of 20 times before the engine would idle. After 10 min at idle I measured the temp of the head at 140 deg F but the car would not run without bucking and stalling. About the time when the oil temp gauge began to register some heat is when the engine began to run normally.

I considered the trigger points, I could understand if one bank of injectors went out but it feels like the entire fuel system stops working, the points are mechanical and could have a temp issue, but to be have no issue outside a small temp range?


I don't have the time to take the FI apart to debug it, I maybe able to in a few days, I will post an update with more info.

Also if anyone else has other ideas or specific knowledge please let me know.
Thanks again,
Mike

[SLITS]

While you replaced the CHT, did you check to see if it was in "range", i.e. ice water, room temp and boiling water?

I just fixed a 2.0 that would stall when cold and thru warmup. When hot it would run fine but idle was a bit low. I tested the CHT and it was out of spec. I then checked the new CHT to see if it was in spec and installed it (an 012, btw). Once I changed the CHT the car ran fine from cold to hot with no stalling. The idle was still a bit low but it did not stall.

I put the idle mix knob midrange to start. 22 clicks from full lean to full rich, so I set it at 11. I then start one click rich (clockwise) and let the engine stabilize and adjust the idle bleed screw for 900 rpm. I usually wind up setting the knob at about 13 clicks where the idle speed is constant and smooth.

This may or may not help you.

[r_towle]

I agree with slits...check your CHT again and make sure its in spec.

Rich

[r_towle]

Problem

After starting when the engine is cold, the idle doesn't come up while the Aux Air Regulator is open (Aux Air Regulator operation has been verified by checking for draw on intake hose). Idle performance is good once the car is warmed up.

Issue

Cold mixture is too lean.
Cold resistance of the cylinder head temperature sensor is too low.

rational

This problem seems to get worse on older cars as the engine wears. There are a couple of solutions. You can "cherry pick" a cylinder head temperature sensor with the highest value you can find, or you can add ballast resistance to the sensor - however, this will affect your mixture across they whole operating range (not recommended). A potential fix is to use a dash-mounted switch to add a ballast resistance to the sensor when cold, then turn it off (shunt it) when hot. I'll update this if I ever find a solution I like!! NEW - another potential solution is to add a spacer to the TS2 sensor to delay the warm-up cycle. See:
http://www.ratwell.com/technical/TempSensorII.html

[r_towle]

2K ohms at 68 deg. F for all but 1973 2.0 L, > 1.2 K ohms for 1973 2.0L

Cylinder Head Temperature Sensor 311 906 041 A 0 280 130 003
0 280 130 012 1.7L
2.0L 1970 - 1973
1974 - 1976 Available new. The ...012 sensor is in short supply. Either sensor should work OK.

[r_towle]

one cure that seems to work, and at the end of the day it is a compromise, is to put in a resistor.
If you go to radio shack the resistors are fairly cheap.

to figure out which resistor you may need, in your specific circumstances, with your specific car, I would suggest putting a POT inline with the CHT.

A POT, or an anjustable potentiometer is availabe at radion shack for about 5 bucks.
get one that goes from zero to 1000 ohms, that is the range you will be testing.
Unplug the CHT and using two small wires with the proper male and female spade bits fastened to them, wire the POT inline.
Start at zero to see of the car start (this is just like it was before you put the POT in place...zero resistance)
Start the car and start turning it up .
It takes maybe 10 seconds or more to even out the idle...so go in small steps and be patient.
Once you have the idle correct, you now need to drive it.
Use tape to hold the POT at the setting you just found that works when the motor is still cold.
Then, take a ride around till it warms up.

Here is where the compromise comes into play.
You may end up turning it down a bit when warm because you will end up running richer than before...

It takes two or three times to really get it perfect...that is two or three cold starts with the know turned down a little until you find the sweet spot that is good for cold start, and good for warm running.

Once you find it, unplug the wires from the pot and using an ohm meter, check the resistance across the two poles of the POT and that is the resistor you need to install in place of the POT.

If you wanna get fancy...run wires to the CHT and place the POT in the cabin and you can adjust it anytime you want.

Most people just figure out the resistor and wire that inline and call it a day.

Using the POT, instead of trying ten different resistors, is just one approach to figuring out what it wrong.



Rich

[SLITS]

Damn Rich ..........

You forgot the instructions for using a VOM.

(IMG:style_emoticons/default/smilie_pokal.gif)

[mgphoto]

OK a buddy of mine loaned me the Bosch EFAW 228 FI tester, I was hesitant about disconnecting 38 year of wires from the ECU.

After running the tests in the Porsche workshop manual the only fault I found was the secondary coil in the MPS, it indicated a possible open circuit (infinite resistance), I pulled the connection multiple time to test the circuit but no go.
I put all the connections back and installed the ECU in the car, started it up and just like that the engine was back to normal.
I am thinking the connections on the MPS, or it is just showing it's age and it's on the way out.

I also found out that my TPS is not adjusted correctly, I will try to attend to that next week.
Well tomorrow morning will be the real test.

I will post more info as I get it.

Thanks to all who wanted to help.
Mike

[r_towle]

what, 40 year old plugs got dirty...I cant believe it.

rich

[r_towle]

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