Uneven running

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paulsv8manta
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Uneven running

Post by paulsv8manta »

Hi guys, finally managed to put 150 miles on the engine, but now its developed a problem I cant seem to solve!
Starts up fine, tick over nice and steady but when I raise the revs to over 1500 rpm it starts to stutter.

When I rev the engine to around 3000 rpm it seems to almost stall fuel pressure jumps from 35 psi to 40 psi. So I believe it's an electrical problem so I've changed the spark plugs, no difference, changed the coil no difference.
Not sure where to go from there anyone have an idea?
The engine is a 3.5 efi flapper, the ECU is the old 4cu.



DaveEFI
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Re: Uneven running

Post by DaveEFI »

Fuel pressure shouldn't change with revs, as it is controlled by engine vacuum. And if you are revving the engine in neutral with the car stationary, the vacuum will be high so it should be reading more like 28 psi. So check the vacuum pipe to the regulator and the reg itself.
Last edited by DaveEFI on Sat Oct 20, 2018 8:24 am, edited 2 times in total.
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ChrisJC
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Re: Uneven running

Post by ChrisJC »

Fuel pressure should be between 26 and 36psi, varying with manifold vacuum.

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richardpope50
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Re: Uneven running

Post by richardpope50 »

(Actually this seems a bit like the same problem as my posts regarding a lumpy 3.5.)

Could it be the dizzy? Can you swap this?
Richard.
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paulsv8manta
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Re: Uneven running

Post by paulsv8manta »

Ok guys, I think I found the cause but not the reason? or should that be the other way round? Anyway thought I'd take a wild stab in the dark before the long arduous journey of checking every sensor etc. I disconnect the ECU, reconnected after 5 mins and started it up hey presto everything working great!

So I took it out for an 8 mile run then hit traffic temperature started rising electric fan cut in and stayed on, 25 mins later cleared traffic stopped at a set of lights and the engine started playing up again and suddenly cut out. Started up ok but engine was struggling and wouldn't let me go over 1500 revs, felt almost like pulling the choke right out, and it kept cutting out every time I stopped. Managed to get home let the engine cool off and pulled the ECU again 1/2 hr later connected it all up started straight up and running with no problems!

Now I'm thinking its an overheating issue, the engine bay is quite compact has an extensive sump tray and the only air flow is through the radiator around the engine and then under the car.
The plenum is so hot you can't leave your hand on it, the air inlet is taken from an external source (through nearside wing with scoop) so nice and cool so I'm thinking the trapped heat of the engine is maybe heating the air which is affecting the fuel? If this is the case not sure which sensor this would affect.
Really appreciate any thoughts on this.

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Re: Uneven running

Post by DaveEFI »

Intermittent coolant temperature sensor or wiring to it? If that goes open circuit, the ECU goes fully rich as for a cold start.

To check both sensor and wiring, with the engine hot and in this fault condition, unplug the ECU had measure the resistance between pin 13 (black/slate) and pin 5 (black/white) or ground.
Should be 150-200 ohms at a coolant temp of 100C. For reference, the reading would be about 2500 ohms at 20C
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Re: Uneven running

Post by unstable load »

paulsv8manta wrote:
Sat Oct 20, 2018 6:04 pm
I disconnect the ECU, reconnected after 5 mins and started it up hey presto everything working great!

pulled the ECU again 1/2 hr later connected it all up started straight up and running with no problems!
Could you extend the loom and move the ECU into the cabin, where it'll be cooler?
Cheers,
John

paulsv8manta
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Re: Uneven running

Post by paulsv8manta »

Hi John the ecu is positioned in the drivers footwell.
Thanks dave i’ll Check that the sensor out.

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Re: Uneven running

Post by DaveEFI »

Other quick way to check for a CTS fault is to unplug it with the engine hot. That should send the mixture very rich, and make it stall at idle. If no difference, do the checks.
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Re: Uneven running

Post by GDCobra »

Pity this isn't 14CUX Hotwire, if it was you could see the sensor readings via Rover Gauge.

paulsv8manta
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Re: Uneven running

Post by paulsv8manta »

Cut in a couple of vents to the bonnet this has helped immensely I’ve had no problems since fingers crossed 🤞.
Thanks guys

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