I have a warm start issue on my '90 240 wagon. Cold it fires up every time. After it gets hot it will crank and crank but never fire. I replaced the fuel pump relay. Now it has been suggested that I replace the crank position sensor. I will gladly, but I don't get why. There is just one sentence related to the sensor in the bentley. What is this thing and what about the engine temp would change its operation?

Hey,
My only experience with the exact same situation(87-740) was solved by replacing the temp sensor, the one under the intake near firewall. Just something else to think about. Probably other things that cause this as well. You can check temp sensor at the computer, unless you can get to it under the intake. check the bently manual, or maybe someone else will have more information. Maybe theres a code for it. Did you check codes? I know there's one for the crank sensor. I have 90-240 also,and my crank sensor failed not to long ago.

Good luck

Larue

If the insulation at the sensor is cracked you should replace it, if it is not bad now it will be later.

Make sure the fuses are clean and tight in the holders.
Dan

thanks all. there are cracks at the cps plug. i can see the wires. i will order one up. and check other things too. I guess i figured the car would not run unless both pumps were working. And I put a new relay in. And it runs so well. The temp needle has to drop all the way down in order to start again. Can take up to an hour.

The power stage / ignition amplifier may be the problem, it fails when hot and works when it cools off. Try a fresh coating of thermal grease and or replace it with a known good used one.
Dan

1990. I think that still has the 25 Amp fuse on the Drivers side Fender.
See how clean the spade tabs on the fuse are and how clean the mating Fuseholder is.
Bad connection there can cause resistance. that fuse feeds the Fuel pumps through the Relay you replaced


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"... what about the engine temp would change its operation?"

The above passage /might/ indicate that it is your crank position sensor that is the villain. It is a semi-conductor, and typically for a failing s-c is that it will fail as it gets warm. I'd consider getting a new one - if you have never replaced it you are on borrowed time anyway. They are not expensive, but there are a few things to consider when replacing them. Shout if you choose to go this route and want some advice.

Erling.
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The CPS, crank position sensor, in LH2.4 is the device that tells the ignition system when to fire. It is quite common for the insulation on the pigtail between the senor and the plug to break or split, allowing moisture to enter the harness and eventually damage the sensor. I've also seen mis-routed pigtails chafed through until wires are touching something metallic.

While a bad CPS can certainly cause a no start, and temperature may be a factor, you've said nothing about occasional cutting out, which is also a known symptom. Thus, I'm going to say that your problem may be elsewhere.

What were the results of the relay change? How much cranking do you have to do warm to get the car to start? Do you smell gas? Have you verified that both pumps are working? When the car runs, is the main pump under the car noisy and uneven? If so, that's a good sign your in-tank pump may be out. With the car on, insert a tube into the filler neck of the tank and listen for the in-tank pump's steady buzz/hum. Does the problem occur only when the car is below 1/2 tank? If so, the pickup connector hose in the tank may be bad.

If you are sure both pumps are working, and you have ruled out any problems with the CPS's pigtail, a little more diagnostic work is in order. Next time the car won't start hot, check the plugs and see if they are gas fouled. A bad engine coolant temperature sensor (about which I'm about to post elsewhere) can cause all sorts of problems, though most common is too rich when cold--with gas fouled plugs. However I have seen them go the other way too. The ECT is the rear-most of the two temp sensors, located under the #3 intake runner. It's a good idea to make sure the plug is intact and that the contacts aren't damaged.

Check your fuel pressure regulator. Quickest check is to pull off the vacuum hose on the front of the capsule and see if there is dampness in the nipple or the hose. If hot, the FPR is sending all the fuel back to the tank, there will be insufficient working pressure to start the car.

Next step would be to check the check valve at the main fuel pump outlet. This is a very common source of hard hot start problems. When the injection system does not hold the recommended "at rest pressure", the car is hard to start hot.

That should give you a few things to experiment with. Post back, I'm sure the board will help you find a solution.

David,

I recently replaced my coolant temp sensor on 1990 240 since I had a 224 and bad mileage (don't remember if I checked for additional codes).

3 months later: bad mileage, funny exhaust smell, occasional engine cutout for a second when letting up on/pressing down on gas.

Have codes 224 (again) and 214. Going to check ECU terminals 10 and 20 per Bentley as part of speed sensor test (page 280-12). Possibly my new coolant sensor went bad, I don't know. Not sure if these codes go hand-in-hand.


Steve

Hi,

The crank position sensor's connector is at the back of the firewall, probably near the valve cover. The black cable from it runs down in back of the engine and the sensor itself is on top of the bell housing. Get a flashlight and shine it in down behind the valve cover towards the bell housing and you will see it connected down there with one 10mm bolt.

Check the archives and the 700/900 FAQ for details about the sensor.

Near the connector there is either a yellow or white stripe around the cable insulation. What color is the stripe? Also, if any of the black insulation is cracked with the inner wires showing, replace the sensor.

Travis