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Misfire Under Load


wowblaauw

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All, thank you so much for your responses. Ok, coil pack replaced with (admittedly not known working) HT leads with the same result. Engine fired and runs very smoothly. Got it to 60d and applied throttle only to get an almost stall a few back fires and the rough revs. Logic tells me that if this coil pack and leads were good then I would get the same results but doubt it would have the exactly the same fault?

 

Spark plugs are new - same result

 

TPS is new and not over tightened

 

Just one thing though; the engine loom to ECU had an issue with not firing the ignition circuit. While I realise this may be a long shot, is there anything in this. The issue was with connection from pin 4 to loom block not making a good connection. A bent pin later and I had a spark and where I am today.

 

Checking earths now

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All, as a quick follow-on, with respect to knights_templar's post. He is quite right I have had loads of issues following a major upgrade over the winter which has dragged on for some months now, being beset by issues mostly created by my good self I must say - age, experience... Anyhow, it would appear that I am getting to the end game. Following another weekend of garage torture I found that the misfire under load was due to fuel pressure. At 3 bar it misfires and at 4.4 bar is perfect. Any thoughts on why this is would be will of course be gratefully received. My map is tuned for 3bar with injectors of 300cc. (Map supplied by GBS to get the engine going ahead of RR) My injectors are standard Ford 200cc - would this be the cause, a mismatch between injector and map? or am I barking again... I've also dropped an email to the good people at Emerald and hope to have a response later on a more suitable map.

 

I would however like to thank you all for your input.

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Ha, I started typing up a post yesterday about fuel pressure and then figured you'd had enough people chipping in, and never posted it X-)

 

All makes perfect sense. Your map was tuned for larger injectors at 3bar, but you've downsized the injectors so at 3bar so you're not getting enough fuel (and possibly atomisation/spray differences, depending on what the standard pressure is for those injectors). By upping the pressure to 4.4bar you're kind of compensating for the lower injectors by making them squirt more fuel in for the brief period they are open - the higher pressure forces more fuel through.

 

Using one of the various injector size calculators out there on the internet (http://www.gtsparkplugs.com/InjectorSizeCalc.html)

 

3 bar @ 263cc/min will give 200bhp at 90% duty cycle, so a 200cc injector won't cut it.

Go up to 4.4bar and 240cc is the recommended size.

 

I've personally run a low injector duty cycle myself, say 80%.

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Craig, The GBS maps were done on STANDARD Ford injectors, the 310cc in the comments field comes from the base map that Dave at Emerald modified into the GBS maps. As long as the injectors are standard Zetec 2.0 ltr the map is fine.

 

As brumster says, increasing the pressure overcomes a lean mixture, but there are other reasons for a lean mixture, and I suspect the throttles are not aligned with the map. Setting this is a much cheaper remedy, than changing injectors.

 

I suspect also that the map comment also give the map as being for Jenveys, am I right?

Edited by knights_templar
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'but my thinking is that you aren't getting enough fuel so when you open the throttle...'

 

Guess my thinking was right ;)

 

admittedly had no idea why you weren't getting enough fuel but i am glad you got to the bottom of it.

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  • 2 weeks later...

All, apologies for not getting back to you sooner and thank you all for your advice etc. It would seem that I have an issue with the new fuel lines I ran over the winter. I jacked up the fuel pressure and settled on 4.4 bar and the misfire disappeared. I'll be stripping out the new lines in a couple of weeks and will run an airline through them - have to fix the compressor first. Engine dies under load would you believe, seems the cap is failing, new one ordered so waiting for it to arrive. Will update you all soon.

 

That said I have a new issue, see my new post, beers, Craig

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am i correct in assuming that running higher pressure will alter the spray pattern? i assume it's like a garden hose if you turn up the water pressure on a jet it will go further but still narrow jet. On an injector i assume you don't want the jet to be too long and narrow as the fuel won't be mixing and the jet might be hitting the side walls of the intake.

 

so bigger injectors i assume put the same amount of fuel as a smaller injector at higher pressure, but in a better pattern? and therefore more power?

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am i correct in assuming that running higher pressure will alter the spray pattern? i assume it's like a garden hose if you turn up the water pressure on a jet it will go further but still narrow jet. On an injector i assume you don't want the jet to be too long and narrow as the fuel won't be mixing and the jet might be hitting the side walls of the intake.

 

so bigger injectors i assume put the same amount of fuel as a smaller injector at higher pressure, but in a better pattern? and therefore more power?

 

there may be a slight change in pattern but I don't think it is significant.

 

The "misfire" we have assumed, is due to a lean mixture, this would be borne out as the increase in fuel pressure has reduced or stopped the problem.

 

The Zetec setup that Craig has is one of the simplest. The TPS tells the ECU the throttle position, the ECU looks at its map and fires the injector to give a set quantity of fuel for that throttle position. If the map was constructed using 3 bar fuel pressure, increasing the fuel pressure will allow more fuel for the same injector firing.

 

The unknown here is the quantity of air entering the engine for a given TPS signal. You can calibrate it but that only tells the ECU the start and stop position, if you have the throttles to far open when you do the basic calibration then you have more air than the ECU will Know about.

 

Which is why, I believe, the problem can be corrected by "setting" the start of the map.

 

Could be wrong, but its a lot easier than other suggestions. RR will of course be best, and will have to be done with the map likely to have been supplied with those throttles.

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thanks for explaining that. I figured (wrongly) that you were saying the injectors shouldn't be run at that pressure as they weren't designed to run at that pressure, but that all makes sense. i guess that's the pay off with injection, more power but more complications.

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Many thanks all, and again special thanks to knights_tmplar et al. You are right, the car did run fine previously - I was warned, several issues waiting for the opportune moment to fail, notably the ECU - fluid damage and a misfire to pot 3 and a partial corrupt map caused havoc. Once past all of these it seems now that a cheap Chinese adjustable fuel pressure reg has caused some miss readings and following a hike in fuel pressure the misfire under load disappeared. I'll be replacing this with a new part soon. Another lesson to learn and I preach it almost daily - "you only get what you pay for".

 

Thanks again fellas, all very much appreciated.

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  • 4 weeks later...

Ok, here’s what I’ve figured. When I had the car up on its stands last year to re-run the new electrics I also decided to replace the cooling and re-run some of the fuel lines. Cooling is now very happy. Fuelling however is not. Before I start with where I am I did replace the ht leads, plugs and coil pack but without change. The original builder ran a mixture of copper piping (all the same sizes) with rubber hose and 90* bends which didn’t look the best and was corroding so I changed from the swirl pot to fuel rail with a new bullet filter and HP pump with what I thought was an-6 lines. Actually it turns out that I was supplied an-8 pipe which I fitted with an adjustable regulator. At first when I discovered the misfire under load it was due to not enough pressure - slight misread on the gauge by me. Then when running the car I now get a lack of power- I mean don’t get me wrong the Car does go, just not that quickly for around 4 miles and then misfires until it stops. I leave it off for a bit and it then fires up and off I go again. I’m probably answering my own question here but do you reckon it’s because of a pressure drop or starvation or account of the change in line sizes? Oh, the HP pump sounds like it wants to take off due to the noise it’s making. I have changed the pump but not the filter. Ideas on this would be gratefully received. Thanks in advance, Craig

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