Author Topic: G40 rough as a badger's arse on throttle at low revs  (Read 205 times)

Offline lectrotek

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G40 rough as a badger's arse on throttle at low revs
« on: July 29, 2019, 01:36:42 pm »
Hi all,

I'm in a right pickle so any help would be much appreciated.

I'm due to hillclimb my G in a couple of weeks but since I got it through MOT it's horrid.

Bit of backstory: Bought the car a year ago. Ran rough on idle and had very little power so got the charger re-built at Jabba inc. full service. Performance was like night and day but was still lumpy on idle so unplugged the lambda.

Ran better (still not great) until MOT time last month. It failed on a whole load of things, one being emissions which were unsurprisingly through the roof. To get through MOT had to get a new lambda sensor as the old one was fucked apparently when they plugged it in, and fit a new cat as the one on there had been obliterated.

So, after much expense (this, welding, steering column, etc.) it got through MOT and felt ok. Drove it 200miles to Devon and back, no probs.

After sitting for a week or so last week, went to take it out and all hell broke loose. Wouldn't start (probably battery) so ended up bumping. When it turned over, I couldn't keep it alive. It was idling OK but as soon as I feathered the throttle it'd die. Bumped it again and again with the same result. Managed to limp it back into my driveway with the help of my gf and dad pushing it for a final bump.

Has anyone else experienced this?

The only course of action I can think of is unplugging the lambda, and replacing the cat with a straight through pipe.

This is driving me nuts! 😬

 

Offline rml2711

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Re: G40 rough as a badger's arse on throttle at low revs
« Reply #1 on: July 29, 2019, 03:24:00 pm »
Hi chap, I would suggest before you start the heavy work, charge the battery...if it's not able to start the car the voltage may not be enough to keep it running.
My alternator needs a few revs before it starts to deliver a decent output,but it fluffs and splutters when you rev it up.
Once the alternator is delivering power the engine starts to run much more smoothly....might just be mine that does that...but it's an easy enough check to do with yours.

Offline hayesey

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Re: G40 rough as a badger's arse on throttle at low revs
« Reply #2 on: July 30, 2019, 10:08:22 am »
yes the first thing to sort is the battery.  If it's putting out low voltage then the ECU will get the hump, they aren't very tolerant of lower voltages.

Offline lectrotek

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Re: G40 rough as a badger's arse on throttle at low revs
« Reply #3 on: July 30, 2019, 10:45:40 pm »
Cheers fellas.

Charged the battery overnight and checked the output in the morning. It's solid. No issues there.

Connections at each terminal were a little loose to tightened them up. Car starts, no problem at all.

Issue does not look like it's battery related.

Do you think it could be ecu?  I bloody hope not  :o

 

Offline hayesey

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Re: G40 rough as a badger's arse on throttle at low revs
« Reply #4 on: July 31, 2019, 09:29:37 am »
I think it's unlikely to be the ECU but I have had one die on me before leading to weird running issues, spent a lot of time diagnosing it and eventually tried a spare ECU I bought out of desperation and it solved it.  But I still think there's other things to check first given that it'll cost you about £100 for a 2nd hand G40 ECU.

So as it is now, it'll start fine and idle fine but as soon as you press the accelerator it just dies?  And there's a good > 13v at the battery terminals when idling?  Does it just do this every single time you touch the throttle? Is it possible to carefully press it and the revs will rise slowly or does that cut right out?  Is it possible to stab it down and it'll splutter but then pick up and rev or does that just cut out?

I would start like this:

- check the ignition timing (will need a timing light, do you know a friendly mechanic you can borrow one from?)

- check/change the coil, plugs & leads.  Unless you have access to spares this means spending some money unless you have some lying around (that aren't also old and knackered).

- check the dizzy: cap & rotor arm first.  They're not expensive and worth replacing anyway if they are old.  Then the dizzy itself, well more accurately the hall sender but it's cheaper to just buy a 6n multipoint dizzy in full (note, the earlier single point injection ones are no good).

- check the condition of the wiring loom, especially the earth points.  Visually check but you can also do a continuity test between all the pins on the ECU connector and the sensor connectors.  There are some Autodata scans in one of the FAQ sections on here with the pinout of the ECU connector in.


Offline markg40T

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Re: G40 rough as a badger's arse on throttle at low revs
« Reply #5 on: August 09, 2019, 12:22:08 pm »
i would also check the resistance of you CO pot on the top boost pipe as this is used to change the richness of the engine on tick over. there is a thread on here that tells you the correct ohms to begin with then you can adjust this at an mot station to correct it to get it to the desired reading.


Offline lectrotek

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Re: G40 rough as a badger's arse on throttle at low revs
« Reply #6 on: September 09, 2019, 12:51:36 pm »
I think it's unlikely to be the ECU but I have had one die on me before leading to weird running issues, spent a lot of time diagnosing it and eventually tried a spare ECU I bought out of desperation and it solved it.  But I still think there's other things to check first given that it'll cost you about £100 for a 2nd hand G40 ECU.

So as it is now, it'll start fine and idle fine but as soon as you press the accelerator it just dies?  And there's a good > 13v at the battery terminals when idling?  Does it just do this every single time you touch the throttle? Is it possible to carefully press it and the revs will rise slowly or does that cut right out?  Is it possible to stab it down and it'll splutter but then pick up and rev or does that just cut out?

I would start like this:

- check the ignition timing (will need a timing light, do you know a friendly mechanic you can borrow one from?)

- check/change the coil, plugs & leads.  Unless you have access to spares this means spending some money unless you have some lying around (that aren't also old and knackered).

- check the dizzy: cap & rotor arm first.  They're not expensive and worth replacing anyway if they are old.  Then the dizzy itself, well more accurately the hall sender but it's cheaper to just buy a 6n multipoint dizzy in full (note, the earlier single point injection ones are no good).

- check the condition of the wiring loom, especially the earth points.  Visually check but you can also do a continuity test between all the pins on the ECU connector and the sensor connectors.  There are some Autodata scans in one of the FAQ sections on here with the pinout of the ECU connector in.

Really appreciate this mate. Checked all this, to no avail.

Offline lectrotek

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Re: G40 rough as a badger's arse on throttle at low revs
« Reply #7 on: September 09, 2019, 12:52:32 pm »
i would also check the resistance of you CO pot on the top boost pipe as this is used to change the richness of the engine on tick over. there is a thread on here that tells you the correct ohms to begin with then you can adjust this at an mot station to correct it to get it to the desired reading.

This is interesting. will look into this.

Offline lectrotek

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Re: G40 rough as a badger's arse on throttle at low revs
« Reply #8 on: September 09, 2019, 12:55:26 pm »
COpot aside, it seems like the aux air valve is buggered.

 
Am trying to track down one of these but they seem to be like hens' teeth.

 

All out of stock on various parts sites.

 

Anyone got one lying around or know where i can get another?

 

Cheers,

 

Tim

Offline hayesey

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Re: G40 rough as a badger's arse on throttle at low revs
« Reply #9 on: September 09, 2019, 01:55:10 pm »
don't think they've been available new for a few years now.  When the cars up to temp, the valve should be shut.  You could potentially test by removing the valve & blocking up the pipes going to it.  It'll should run ok when hot.