Firstfives.org

I'm tracking down a cold start problem, and I think the culprit may be the coolant temp sensor that sits next to the aux air valve (my '79 has the "Jules Verne device" under the intake runners). I ordered a replacement and installed it, but found the cold start problem to be even worse upon reassembly.

Granted, I may have put something together wrong. I'm looking into that. My question for the group's wisdom is this: are these two sensors, both listed as part 13621357414, the same? Here is a $62 part from BavAuto that looks just like the sensor I removed. And here is a $16 part that I ordered from AutohausAZ. Note that the Autohaus sensor has a blue connector. Are they compatible? Same specs? Can anyone confirm that the Autohaus sensor works for them?

Thanks!

Also: big shout out to Robert Bondi for this post that saved my bacon & sanity when I broke that three-way hose.

Do you know if the cold start valve is working?

With the engine cold, pull the cold start valve and put it in a jar where you can see it from the driver's seat. Turn the key to "start" and see if/how much fuel comes out of the valve. It should be a fine mist that lasts from a fraction of a second (if it's hot out) to 8 seconds (if it's cold out).

The sensor near the aux air valve (I believe) should be a "time/temperature" switch instead of just a temp sensor.


Did you look up your part numbers on realoem.com ?

socalfiver wrote:Do you know if the cold start valve is working?

Good point, thanks. I should verify that. My problem has been with a very weak idle after a cold start which led me to suspect the coolant temp sensor based on other posts I've read here. This system changed so much every year there's no precise info in Haynes. I will check it.

I did remove the aux air valve and give it the freezer test. Seemed fine.

The sensor near the aux air valve (I believe) should be a "time/temperature" switch instead of just a temp sensor.

There is a time/temp switch there too. Brown Bosch connector, sticks straight down out of the box the aux air valve screws onto. The sensor I replaced, the coolant temperature sensor, has a white connector and installs horizontally on the aft of that box.

Did you look up your part numbers on realoem.com ?

bmw mobile tradition. Isn't it the same database? My car has a 6/79 birthday.

See my second to last post here. And be sure to follow the link in the post.

Also look for my post at the bottom of the page regarding grounds here.

Lenny D. wrote:See my second to last post here. And be sure to follow the link in the post.

Thanks, Lenny! Yours were definitely some of the posts I consulted when I was doing my homework on this problem, and led me to suspect the coolant temp sensor. But as you say, grounds. Always be checking your grounds. You mentioned the CTS and the ECU were grounded "on the right side intake manifold nearest the firewall." Is point A that connection or is it somewhere else?


I'm glad to know these razzm frazzm brittle plastic connectors can be replaced without too much drama, since they crumble to dust in my clumsy hands.

I ended up picking up a coolant temp sensor from the local parts store for around $15 instead of ordering the expensive unit from BavAuto.

So far, so good, but who knows.

Adam Gravois wrote:

Lenny D. wrote:See my second to last post here. And be sure to follow the link in the post.

Thanks, Lenny! Yours were definitely some of the posts I consulted when I was doing my homework on this problem, and led me to suspect the coolant temp sensor. But as you say, grounds. Always be checking your grounds. You mentioned the CTS and the ECU were grounded "on the right side intake manifold nearest the firewall." Is point A that connection or is it somewhere else?

Indeed, point A is the 'sacred (L-jet) ground'.
Be sure to replace the terminal, cut the two wires back to fresh wire and SOLDER them onto the new terminal. L-jet ground problem solved forever. Bosch also says there is to be nothing else connected to this ground point. And follow the engine ground back to the battery negative checking for virtually 0 resistance.

JoshInAtlanta wrote:I ended up picking up a coolant temp sensor from the local parts store for around $15 instead of ordering the expensive unit from BavAuto.

So far, so good, but who knows.

That's what I was hoping for with this $16 Bosch unit from Autohaus. How did you know if yours would put out the right voltages? Is this part so generic that they are generally all interchangable?

At 68 deg. F, 2-3K ohms.

Lenny D. wrote:At 68 deg. F, 2-3K ohms.

Thanks for the help. I'm going to go back over that stuff this morning.

Update: Cold start valve functions correctly. Old coolant temp sensor delivered 1600 ohms resistance at 95 degrees; I'll test the new sensor and replace the ground terminal when I get back to it. I have to set this repair aside for a week or so. At least it runs.

I messed up the voltage drop testing, so I'm not sure if the fault lies there or not. Is it possible that a voltage drop too small to detect on the multimeter would still cause this kind of gremlin? I still don't fully grok electrical diagnosis... someday I will.

It's not that resistance will be too small to measure it's that L-jet operates on microvoltage so anything that is not operating as originally engineered (hard to accomplish 30 years later, particularly without good maintenance or rebuild of electrical components) will be very noticeable as engine management is now incorrect. And the grounds are the worst culprits, since they can ground 'somewhat' and for a long time before complete failure, but not good enough to have the system operate correctly.

As part of my B35 conversion I took apart the 3.0 to recycle it and was astounded at how awful the injector hoses, seals and in general all the flexible parts were. It was a wonder the car even ran, and I won't even mention how lucky i was not to have an engine fire. It's what you can't see that you should put on the list for attention. Those injector hoses shattered when I touched them. Air leaks are the performance item, polymer deterioration is gonna catch up with you sooner or later. Sclerosis of da hoses.

When I overhauled my intake, I changed all of the injector seals and hoses. I found the hoses to be okay, but the injector seals had turned into hard brittle plastic, rather than supple rubber.

Yeah, I have lots of sclerosis of the hose and rubber! I am very wary of what I touch now unless I have the replacement part in hand. Fuel and vacuum hoses in particular.

Blaise, I saw your overhaul pics. Terrific stuff, very educational.

Philosophical question for the learned: if I replace ground connectors, is it possible that cleaner microvoltage signals will require a tweak to the idle settings which have been tuned to work with the old, degraded signal? Which settings would those be likely to be? The dial in the AFM?