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Discussion Starter #1
Further to my other post my ST is running on one Cylinder. I did the belts this year and the bike started and ran fine. I have since checked them again and they look OK. After a period of leaving the bike the front pot will not fire


  • Battery is in new and in good nick and is backed up by a booster
  • Checked and swapped Plugs and Injectors
  • All loom lines to the above measure well in Ohms and voltage
  • TPS is new and set up
  • Compression is good
  • ECU loaner was tried - same result
  • Injectors are spitting fuel
  • Valves are moving
  • Coils checked and swapped
  • HT leads checked 750 OHM one side 1K OHM the other. No cracks or splits
  • Disconnecting Front HT lead bike starts and runs on that Cylinder. Disconnecting rear HT lead bike will not run -nothing.
 

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Johnny, have you verified you're getting spark to the dead cylinder? Pulled the plug, ground it to the head and turned it over? Don't mean to be silly, but if you have one faulty plug and you swapped them, you just moved the dead cylinder to the other side. Actually that goes for any swap (injectors/plugs/coils); still the same cylinder that's dead?

Man, good luck
Scott
 

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another way is squirt starting fluid into intake and try. if spark is ok it will catch and run for a revolution or 2. I believe it's ether and contains O2 so getting a correct mixture is not an issue.

quick and dirty way to check spark.

bob
 

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I didn't see where you swapped the plug wires. Hard to really tell anything by measuring resistance on these, IMHO. They aren't a copper strand wire, they are suppression core wires and can certainly fail. Maybe grab a used cheap set off eBay to test with. If that's it, buy a good set of Magnecor wires and be done with it.
 

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Discussion Starter #6 (Edited)
Johnny, have you verified you're getting spark to the dead cylinder? Pulled the plug, ground it to the head and turned it over? Don't mean to be silly, but if you have one faulty plug and you swapped them, you just moved the dead cylinder to the other side. Actually that goes for any swap (injectors/plugs/coils); still the same cylinder that's dead?

Man, good luck
Scott
Yes Scott I also purchased one of those plug indicators. When I did my swaps the same cylinder did not fire

another way is squirt starting fluid into intake and try. if spark is ok it will catch and run for a revolution or 2. I believe it's ether and contains O2 so getting a correct mixture is not an issue.

quick and dirty way to check spark.

bob
Cheers Bob I shall give it a go

I didn't see where you swapped the plug wires. Hard to really tell anything by measuring resistance on these, IMHO. They aren't a copper strand wire, they are suppression core wires and can certainly fail. Maybe grab a used cheap set off eBay to test with. If that's it, buy a good set of Magnecor wires and be done with it.
Yes Thought that too, I have ordered new HT leads -Cheers Dan

typical dead ecu issue for the early st4s.
Tried a swap Brad with an ECU I borrowed, still the same

Thanks for the ideas guys
 

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well, at least that's ticked off the list. pity tho, as it's an easy one. dropping a cylinder otherwise is pretty uncommon.
It might be time to start talking about sensors.
Would say a bad coolant sensor (false high) trigger a limp home mode that doesn't clear? Would any other wacky sensor trigger a limp home mode?

I didn't see where you said which ECU you have. Is it the 5.9M?

(Sorry Brad, I meant this to be directed back to Johnny. New guy fat finger)
 

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For the 2002 ST4s, it will be the 5.9 ECU. These are pretty simple FI units. CPS trigger is used to time spark and manage fuel for both cylinders. The other sensors are air pressure, air temp and coolant temp. All of those are going to effect fuel mix for both cylinders. He's been running on one cylinder pretty consistently. Loosing one cylinder, if electrical and not fuel or compression related, is more likely the ECU (which has spark trigger "igniter" for each cylinder), coil, plug or plug wire.
 

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For the 2002 ST4s, it will be the 5.9 ECU. These are pretty simple FI units. CPS trigger is used to time spark and manage fuel for both cylinders. The other sensors are air pressure, air temp and coolant temp. All of those are going to effect fuel mix for both cylinders. He's been running on one cylinder pretty consistently. Loosing one cylinder, if electrical and not fuel or compression related, is more likely the ECU (which has spark trigger "igniter" for each cylinder), coil, plug or plug wire.
He already tried an ECU swap, no joy.
 

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He already tried an ECU swap, no joy.
Read that, just laying out the possibilities in general. Reading his post above, I think he already found no spark on the bad pot. Given that he has already swapped ECU, coils and plugs I felt he should try the wires. There is no "limp home" mode for these bikes that I am aware of. The FI on this generation of ST is open loop. No O2 sensor feed back. Not much more than an electronic carburetor really. Some fuel trim adjustment based on a few sensors, but that's it.
 

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Discussion Starter #12
I replaced all sensors last year but like others can't see how it will effect one Cylinder. Checked the Fuel pump/injector/Coil line too can't see any volt drop or continuity issues.

Started thinking about silly stuff now.

Blocked Exhaust, broke exhaust valves?? This would be picked up by the compression test?
No earth on the front cylinder or blocked spark plug hole??
Something shorting in the front cylinder spark hole?
Excessive fuel mix in one cylinder?
 

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Read that, just laying out the possibilities in general. Reading his post above, I think he already found no spark on the bad pot. Given that he has already swapped ECU, coils and plugs I felt he should try the wires. There is no "limp home" mode for these bikes that I am aware of. The FI on this generation of ST is open loop. No O2 sensor feed back. Not much more than an electronic carburetor really. Some fuel trim adjustment based on a few sensors, but that's it.
Is it no spark or is it a cold jug? Not sure which it is? Can you help Johnny?

I wasn't aware of any limp home either, I'm just digging deep cause it looks like Johnny has tried all the other stuff.
What does a false high temp signal do (anything other than turn on the fan)?

My job has been to just keep the blue smoke inside the boxes, What the blue smoke does inside those boxes sometimes evades me .
 

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If this were an old car I would be looking for carbon tracks inside the distributor cap (scrape clean, apply fingernail polish). I know it's not even remotely applicable, but it just reminded me how deep a trouble shoot can go.
 

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If this were an old car I would be looking for carbon tracks inside the distributor cap (scrape clean, apply fingernail polish). I know it's not even remotely applicable, but it just reminded me how deep a trouble shoot can go.
You might be close...aren't the high tension leads tie-wrapped to the frame at some point? I believe SOMEONE in here found a leak (short) at that point. Might be worth checking...
 

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>>>There is no "limp home" mode for these bikes that I am aware of. <<<
I believe there is...my great local tech that did my valves about 5 years ago didn't notice that the TPS was unplugged when he was done servicing my bike. So when I picked it up, he ranted about how lousy it was running. And I had a close friend limp it (I just had Rot.Cuff surgery) back to my house, and he just shook his head saying "it ran very poorly".

Note that it ran "perfect" when we dropped it off for the valve service the previous week!

It took me a day to look at it in my garage, and I found the TPS unplugged. So I believe it was running in "limp-home" mode. Reconnecting it, the bike ran perfect again.
 

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Based on what you have checked I suspect there is a problem with either power or earth on the coil.
I would measure voltage to the vertical cyl coil positive terminal whilst running or attempting to start and compare that to measurement taken on horizontal cyl.
If this checks out, check for a pulsed earth at the negative terminal on the ign coil. To do this remove the neg terminal and attach a test light to the wire , the other end of the test light to the bikes positive battery terminal, now start or attempt to start bike and see if test light flashes once every 2 revolutions. Note: if using LED test light make sure its connected the right way around ie, clamp on the wire and probe end on battery positive.
 

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Discussion Starter #18 (Edited)
Based on what you have checked I suspect there is a problem with either power or earth on the coil.
I would measure voltage to the vertical cyl coil positive terminal whilst running or attempting to start and compare that to measurement taken on horizontal cyl.
If this checks out, check for a pulsed earth at the negative terminal on the ign coil. To do this remove the neg terminal and attach a test light to the wire , the other end of the test light to the bikes positive battery terminal, now start or attempt to start bike and see if test light flashes once every 2 revolutions. Note: if using LED test light make sure its connected the right way around ie, clamp on the wire and probe end on battery positive.
Cheers for this one.I checked earths and attempted to check positive to Coils and Injectors but they are both pulsed. Broadly voltage was the same BUT the led trick should help

From memory the voltages at the Injectors were a little strange, showing -2Volt at ignition and +11 at start. I need to check that as I may have got it wrong..I have done so much
 

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the "limp mode", as people like to think of it, is when the ecu will assign a default value to a sensor input if the sensor input voltage is out of the defined range. usually for a tps it's effectively about 12 degrees. it should cruise really nicely around 120km/h or so. 25 degrees air temp, 65 degrees engine temp sort of thing.
 

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The positive side is not pulsed but the negative is. when the earth is turned on it will cause a voltage drop which will appear to be pulsed especially on a digital multimeter. Voltage drop while running should be less than 1v for a coil.
The same thing happens with injectors which are also negatively switched.
 
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