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Intermittent starting issue from cold

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30 Aug 2021 16:27 #238022 by LesNewell
I recently acquired a 2002 Jimny auto with an odd starting issue. From cold/warm it will either start immediately or crank and not even try (mostly the latter). Once started it runs smooth and quiet. No misfires or any other miss behaving. The check engine light remains off. When it's hot it seems to start every time. I don't currently have an OBD2 adapter so I can't read out any fault codes. I have one on order.

Here's where it gets a bit odd. When it won't start I still have a spark, fuel pressure is just under 50PSI and I can see the injectors are getting pulses as it cranks. It just doesn't fire. If I then give it a sniff of easy start spray it will start straight up. It feels like a fuelling issue to me but the injectors are getting triggered and there is fuel pressure. I don't think the fault is in either the crank or cam sensors. If I disconnect either one it won't spark or fire the injectors. I have to admit I'm a bit stumped on this one. Any ideas would be gratefully received!
 

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30 Aug 2021 16:57 #238023 by LesNewell
Huh, I just found something interesting. I hooked one of the injectors to an oscilloscope. When it won't start the cranking injector pulse width is about 1.4ms, which seems very short. Idle is ~4.2ms and I'd expect more fuel than that when cranking. I managed to catch one good start and cranking pulse width looked like 10ms or more. I only caught a glimpse before it started.

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30 Aug 2021 19:36 #238027 by DrRobin
It sounds like either a ECU fault or more likely a sensor fault, that is not providing a long enough pulse for the injectors when cold.

I take it battery voltage off s good? After that it could be the crank position sensor, giving a wrong position and reduced pulse to the injectors.

It might also be a faulty temperature sensor, the ECU thinks the engine is really warm.

A code reader might help give further insight.

Robin

2020 blue SZ5
Ex 2011 Blue Jimny SZ4
Northumberland Jimny Blog

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30 Aug 2021 19:50 #238029 by LesNewell
Thanks for the reply. The battery's pretty new and I tried jumping it off a big battery. It cranked a little faster but didn't start.

Hopefully my code reader will turn up in a few days. If that doesn't shed any light on the issue I guess I'll have to load up the parts shotgun and throw some new sensors at it.

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30 Aug 2021 20:39 #238033 by Dirtbird

Thanks for the reply. The battery's pretty new and I tried jumping it off a big battery. It cranked a little faster but didn't start.

Hopefully my code reader will turn up in a few days. If that doesn't shed any light on the issue I guess I'll have to load up the parts shotgun and throw some new sensors at it.

Heavy Duty Transfer Box Brackets, 2-inch Black Raptor lift kit, caster adjusters, Rust removed and stich welded new sections. Galvanised Chassis. Body,Engine,transmission swap. Anti Rollbar Roll / Sway Bar / DAVES ARB disconnect kit.Osram Night Breaker Head light mod and white 501 led side lights.

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30 Aug 2021 20:44 #238034 by Dirtbird

Heavy Duty Transfer Box Brackets, 2-inch Black Raptor lift kit, caster adjusters, Rust removed and stich welded new sections. Galvanised Chassis. Body,Engine,transmission swap. Anti Rollbar Roll / Sway Bar / DAVES ARB disconnect kit.Osram Night Breaker Head light mod and white 501 led side lights.

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30 Aug 2021 21:46 #238036 by LesNewell
This one's OBD2 so unfortunately it doesn't have that connector. I spent some time looking before I realised it's OBD2

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31 Aug 2021 20:10 #238079 by Dirtbird
ok are you sure?

Heavy Duty Transfer Box Brackets, 2-inch Black Raptor lift kit, caster adjusters, Rust removed and stich welded new sections. Galvanised Chassis. Body,Engine,transmission swap. Anti Rollbar Roll / Sway Bar / DAVES ARB disconnect kit.Osram Night Breaker Head light mod and white 501 led side lights.

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31 Aug 2021 20:19 #238082 by LesNewell
Well, I can't find the CEL connector and it's got a standard OBD2 socket under the dash in the driver's footwell.

I did some more testing today. I hooked the oscilloscope to the cam sensor and it's output exactly matches the waveform in the service manual. To monitor signals I push a wire into the back of the sensor plug, through the little rubber seal. I prefer doing that to poking things through the wire insulation. I did the same thing to the crank sensor plug and it now appears to start every time. I'm not sure at the moment if it's the sensor that's playing up or the plug but it certainly looks like it's one of the two. I have a new crank sensor on order.

Looking at other evidence I think this engine has been cooked at some point. The heat could have damaged the sensor. I'm replacing the cam sensor as well just to be safe.

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31 Aug 2021 21:11 #238085 by DrRobin
Some good progress there. If fiddling with the connector on the crank sensor then it sounds a bit like a bad connection, the plug might have been damaged with excessive heat.

I guess if the sensor isn’t too expensive then it makes sense to replace it as well.

Perhaps some contact cleaner or even WD40 will sort out the plug?

Robin

2020 blue SZ5
Ex 2011 Blue Jimny SZ4
Northumberland Jimny Blog

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31 Aug 2021 21:44 - 31 Aug 2021 21:47 #238086 by LesNewell
The connector looked pretty clean. I have unplugged it several times during testing and it didn't appear to make any difference. The only thing I did different this time was poking a wire in the back of the connector so I could monitor the sensor while it was plugged in. When the new sensor arrives I'll inspect the plug more closely to see if there is a broken wire then if it all seems file I'll give it a good dose of Electrolube EML. Unlike most contact cleaners it leaves a thin lubricating film that helps prevent corrosion.

Now I just need to fix the rust in the back and the rather worrying little bubbles of gas popping up in the radiator - looks suspiciously like head gasket <sigh>
Last edit: 31 Aug 2021 21:47 by LesNewell.

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29 Oct 2021 21:30 #239733 by LesNewell
Just a quick progress report on this issue. I put it on the back burner while I was doing the welding, head gasket etc. Today I pulled the contacts out of the crank sensor plug and found the wire on one was pretty green and corroded. I don't have any of those contact crimps so I cut the wire back and soldered it to the crimp. It appeared to work but on the test drive the auto box seemed reluctant to change up. When I returned from the test drive it again didn't want to start. I now have an OBDII adapter so I plugged it in and got a whole stack of fault codes, including throttle position sensor. I checked the throttle position readout and it was wandering all over the place, mainly around 70 - 100%.

On checking the sensor there is no connection to the middle pin. The sensor is toast. This actually explains a lot. Most ECUs will stop the injectors if you hold full throttle while cranking. This is to clear a flooded engine. With the throttle reading wandering around randomly it could easily put the ECU into flood clear mode. That would explain the strangely short injector pulses.

The high throttle reading would also put the gearbox into kick down mode, which explains the reluctance to change gear.

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