Running rich AND lean with power boost valve

[Olav Bergman]

Hello All,

When idling I smell fuel, and I suspect idling is not as smooth as it can be.
I am talking about the 2.9 block.
I've been looking at the rolling road results again from last november. I noticed that when idling the car ran about 10% too rich but when above 1500 rpm the car is about 10% too lean over the whole higher rev range.
I have the power boost valve installed and I adjusted it so that the car responds quick to the throttle. (the delay as minimal as possible)
To cure the idling rich problem my idea is to lower fuelpressure a bit on the power boost valve.
But then at higher rev's It is likely that it is running even leaner. that won't be good..

My question: Is there a way to make the mixture at idle leaner and while at the same time enriching it at non-idle revs? (without extra chips etc) Will simply lowering fuelpressure work?

[Barkychoc]

Olav, I don't know much about the fuel pressure regulator, but isn't it supposed to be a rising rate regulator, which from what you're saying sounds like it isn't rising enough as the revs increase ?
I'm not sure how it works or indeed if revs has any impact or if it is more to do with fuel flowing through it?

Chris

Edited by Barkychoc on Thursday 27th December 18:35

[Jed-S]

I think you would better off fitting a fuel pressure tester and see what the readings are because that will be the only way to get more facts to help isolate the cause.

If it was me then this is what I'd do.
It could be that the regulator isn't working fully and giving a constant pressure. The fuel pressure is varied by the vacuum so you do have the vacuum hose connected don't you?
Set the max pressure to 3bar and see if that makes a difference. Do you still have the original pressure reg to try?
It helps when you have a fuel pressure testing kit

One thing to be aware of, the 2.9 engine allegedly does benefit from larger injectors - it is mentioned on some Sierra XR4 forums and someone on here had their S mapped and said it could have done with more fuel than the standard injectors could supply. It could be that what you're seeing at revs is normal.

Are you aware that the idle mixture can be adjusted on the air flow meters? This could get your idle mixture sorted

[Olav Bergman]

Thanks for the reply. I will check the vacuum hoses. Here are the rolling road results. (before timing adjustments) The graph at the bottom is fuelmixture. It is supposed to be at the red line (14)

Edited by Olav Bergman on Thursday 17th January 21:49

[trackcar]

thats a rich mixture not a weak one .. the red line looks like a 14.7:1 reference looking at the scale, and going down from that is rich and up from that is weak. Theres a bit at the start that is very rich, so much so that it's harming hp (you cna see the change in the hp curve) but after that it's very good, maybe a litle righ at 5k revs thereabouts but otherwise nothing wrong at all. I think you're looking at hte fuelling curve from the wrong perspective .. the lower the number the richer the mixture.

[Olav Bergman]

Mmmm, ...if you look at it that way. When it uses 10 instead of 12 "units" of air to 1 "unit" of fuel means less air = rich. mmmm...Yes! Make's sense.
Thanks!
Then I could enrich the idle mixture using the adjustmentscrew on the AFM and also lower fuelpressure a bit on the boostvalve...that might solve it over the whole range..

Edited by Olav Bergman on Thursday 17th January 21:48

[Sweet Thunder]

Actually, the graph doesn't look that bad: the red horizontal line shows the theoretically perfect air/fuel ratio, but the blue horizontal line is where i expect the air/fuel ratio for maximum power. Your graph sits nicely between the two from 2000rpm up till maxrpm. In an ideal world you'd want to get rid of the dip between 1300-2000rpm (you can see that this coincides with a dip in the torque graph), and the mixture at idle seems a bit too weak. fiddling with AFM and fuel pressure won't properly solve this, it would merely move the complete graph up or down the scale.
The 'bulge' at idle and the following dip are probably caused by the fact that TVR used a bog standard engine and ECU, and then connected an exhaust with virtually no back pressure. The only proper way to solve this would be a piggyback or programmable ECU, but it looks like you already have got the most from the system 'as is'.