Misfire and cold start trouble

[BearTrap]

First of all I would like to that everyone with the help on my turn signal conversion to LED lights. You guys are awesome!

Now I am tackling another problem with cold start. A few weeks ago, the bike was running perfectly. Cold or hot the bike starts right up with no more than the first position on the fast idle lever. Slowly it developed a slight misfire. This progressed into cold start problems. As it stands now, when starting cold, it chugs as if gasping for air (you can see and smell the gas coming out of the tail pipe). Once warmed up it idles perfectly at 1k rpm (still a slight misfire at 3k and up). While still hot, I can restart with no problems. Once it cools for a few minutes, it chugs again.
This is a 1990 K75 with 51k on the clock.
Things I have checked so far;
1.    Checked valves: intakes 1=.015mm, 2=.015mm and 3=.025mm. Exhaust: 1=.025mm, 2=.025mm and 3=.025mm
2.    New spark plugs added
3.    Spark plug wires checked for shorting visually and running a ground wire over them.
4.    Switched coils 1-3 then 2-1
5.    Removed and tested all vacuum lines with a vacuum pump
6.    Replaced the o-rings on the BBS’s
7.    Throttle bodies synced
8.    The TPS was set properly
9.    Timing checked
10.  Barn door on the MAF is not binding and opens smoothly.
11.  Checked the air bypass screw on the MAF and it was set to 2 turns from fully seated
12.  Disconnect, clean and reconnect all connections, relays and ground wires
13.  Battery is strong.
14. Radiator fan functions as it should when the engine is hot

This leaves me with one item that I’m not clear on is the temperature sensor. I checked my Clymer’s manual and there is no procedure to test it. I’m not even sure how the connector is removed from the sensor (I don’t want to force it seems stuck).
Any suggestions will be greatly appreciated.

Merry Christmas!

[The Mighty Gryphon]

It's late, so I don't have the time to find the information for you, but you need to search for the pin assignments for the Jetronic control connector.  The temperature sensor signal comes up through that connector, and that is the first place to check because you will not only check the sensor, bur the wiring as well.

Ideally, you want to check both cold and after the engine is at operating temperature.  The sensor has a variable resistance that decreases as the engine gets hotter.  I don't know the exact values, but I think they are something like 2500 ohms cold and about 200 ohms at operating temperature. 

Even if the fan works properly, the engine is controlled off a separate sensor which can be bad.  I think the Clymer manual electrical drawings for the early K75 and K100 models shows the pin numbers on the jetronic plug.  The problem is reading the numbers on the actual plug.

Merry Christmas.

[BearTrap]

Thanks, I'll look into that next week. Merry Christmas.

[Martin]

To test the actual sensor you need a thermometer and a multimeter. Heat the water and check the resistance compared to the temperature. using the attached chart.
K100 Temp Sensor Values.jpg (38.1 kB . 768x464 - viewed 816 times)
Merry Brickmas regards Martin.

[BearTrap]

Do I need to drain the coolant to remove the sensor?

[rbm]

Yes, you will need to drain the cooling system.

[BearTrap]

Thanks Martin, Merry Christmas.

[BearTrap]

Hope everyone had a Merry Christmas.
Here is an update.
·         I pulled the air box and cleaned the contacts on the water temperature sensor
·         Cleaned the contacts at the sensor one reads 2.72k and the other 2.71k @ 17.2degrees Celsius
·         Continuity of the two wires check out and have the same readings as above
·         Cold start is better
·         Fan comes on at 79.7 Degrees Celsius with a resistance reading of .226k at the computer and .212 at the temperature switch.
Still having some misfire around 2k RPM and up see video @

[rbm]

This is a 1990 K75 with 51k on the clock.
Things I have checked so far;
1.  Checked valves: intakes 1=.015mm, 2=.015mm and 3=.025mm. Exhaust: 1=.025mm, 2=.025mm and 3=.025mm

I went back over your list, and noticed the valve measurements.  The BMW spec for valves is:
Inlet 0.15 ... 0.20 mm
Exhaust 0.25 ... 0.30 mm

I see that you've written .015mm when you might actually mean 0.15mm and .025,, when you meant 0.25mm.  If I assume your measurements were meant to be written in this way, then that means #3 intake is 0.05mm over the maximum tolerance.  Can you double check that measurement?  That may be a contributing factor to the misfire.

[BearTrap]

You are absolutely right. Don't now why I added the extra zero. Gonna recheck them this weekend and order the appropriate shim for number 3 if need be.
Thanks.

[wally.fisher]

Hope everyone had a Merry Christmas.
Here is an update.
·         I pulled the air box and cleaned the contacts on the water temperature sensor
·         Cleaned the contacts at the sensor one reads 2.72k and the other 2.71k @ 17.2degrees Celsius
·         Continuity of the two wires check out and have the same readings as above
·         Cold start is better
·         Fan comes on at 79.7 Degrees Celsius with a resistance reading of .226k at the computer and .212 at the temperature switch.
Still having some misfire around 2k RPM and up see video @

A little limp waisted there old boy , wring it's neck a little let's see a little more use of the throttle.


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[Laitch]

Fan comes on at 79.7 Degrees Celsius with a resistance reading of .226k at the computer and .212 at the temperature switch.

That is a problem indicator right there. The fan should start around 103ºC. What were the sensor's values from boil to cool.

[BearTrap]

When I made the video, cold start was 2.72k at the computer and 2.71k at the temperature switch @ 17.2degrees Celsius. Fan comes on at 79.7 Degrees Celsius with a resistance reading of .226k at the computer and .212 at the temperature switch. Temperature reading taken with an infrared thermometer at the base of the water temperature sensor so values will not be 100%

[Laitch]

. . . so values will not be 100%

Well, that part seems accurate, that's for sure. :giggles

[wally.fisher]

Unless I'm having a slight wobble here my recollection is that the radiator temp stat opens at 82 deg C, so how is it your fan starts running before the bike has reached what is normal operating temperatures?


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[BearTrap]

After a rough cold start, I took the bike on a long ride. When I returned and let the bike sit overnight, I pulled the plugs the next morning to take a gander. The problem is with cylinder #3. The plug 3 smells of and covered with fuel. Thought I'd take a closer look at the plug wire #3 again. while bending the wire I found a cut in it possibly for rubbing on #2's cap. Looks like a new wire is in order. This brings a question to mind. has anyone used aftermarket wires like the one @ https://www.euromotoelectrics.com/Ignition-Wire-Set-BMW-K75-BMW-WSK75-p/bmw-wsk75.htm ???

[rbm]

RAM Power Leads are highly recommended if you're looking for aftermarket replacements.

[BearTrap]

Thanks, and they come in red too! :2thumbup:  I've heard of BSK. They make a wicked track bike. Thanks for the tip. Happy New Years

[BearTrap]

I recently replace the spark plug wires with new aftermarket.  Starting is better but I still smell gas from the exhaust and it takes a moment or two number three cylinder to kick in.

Today I decided to recheck the valve clearance at 50,814 miles with a new set of feeler gauges. Here are the results.

• 1=.15mm
• 2=.15mm
• 3=.18mm

Exhaust:

• 1=.28mm
• 2=.20mm
• 3=.28mm

Looks like number 2 exhaust may be the contributing factor but number three cylinder is the one that is running rich.
Three questions:
I definitely need to take care of number two exhaust, but should I adjust number 3 intake as well? Would all intakes measuring .15mm and all exhaust measuring .28mm be an ideal setting for my bike?
Found a BMW tool (11 1 720/722 new) on eBay for $69.00 + free shipping. Think I’m going to order it to make life easier.
Thanks again!
1990 K75 / 50,814 miles

[Laitch]

I definitely need to take care of number two exhaust, but should I adjust number 3 intake as well? Would all intakes measuring .15mm and all exhaust measuring .28mm be an ideal setting for my bike?
Found a BMW tool (11 1 720/722 new) on eBay for $69.00 + free shipping. Think I’m going to order it to make life easier.

If the valve clearances are within spec, leave them alone. Just correct the one that is out of spec. As far as the tools go, Ken Lively's are cheaper and just as effective. Check details in this post. Email him for current information.

[BearTrap]

Thank you.

[daveson]

i wonder if the fuel pressure regulator diaphragm is leaking
it is connected to cylinder three by a vacuum hose
i wonder if there is a change to fuel smell if you disconnect the hose at the throttle body and plug both ends

[BearTrap]

Now that makes sense. A week ago after I replace the faulty spark plug wire I did another throttle body sync. After the engine was warmed up and turned off I removed number three vacuum line and a few drops of fuel came out of the hose. I'm going to try cold starting it with your suggestion. If successful, then my problems point to the regulator. Are the regulators rebuildable?
Thanks for that info.

[The Mighty Gryphon]

Not rebuildable.  But there are usable pre-owned regulators to be had out there.

[daveson]

if fuel came out the vacuum hose, then the regulator is leaking
you could keep riding with the hose plugged until replaced, i guess its only job is to reduce fuel on low rpm deceleration
somewhere i read that some people disconnect it permanently