FI enrichment effects on idle and vacuum.

[Dave WM]

My 75 starts well drives well, gets good gas mileage, BUT has always had a slight miss sound at low idle.

I say low idle since when warming up the AAR keeps the idle higher. After it warms up it drops into a recurring miss.

I tested things like compression (fine) spark timing (fine) fuel pressure (correct) and vacuum (a little low 17hg at sea level).

Since it only seems to miss at idle and reading up on mixture level having an effect on vacuum (I presume do to less than an optimal burn, reducing overall efficiency resulting in lower rpr for a given throttle opening) I decided to try the temp sensor pot mod. I used a 500ohm WW pot, added some bullet connectors for easy of series insertion into the wire harness. full CW adds 500 ohms to overall resistance now. I started the car and drove it to warm up, while idling I turned it up. there was a noticeable pick up of RPM, maybe 100 max and the slight miss became almost unnoticeable. Further testing had the vacuum go from 17 to 18 (I could dial it back with the pot 0 added ohms 17 500 added ohms 18hg).

I suspect that my OE injectors may not be optimizing the spray patterns perhaps even a single one is off (the slight miss) that is compensated by the added fuel. I don't consider this a perm fix, more a diagnostic tool.

Since I don't have an O2 sensor I don't have a good way of directly reading the fuel mix, but I do have one of those colortune clear plugs on the way.

The plan is to try it in each cylinder and see if I can notice an issue. I tried a load test but it did not show any real problem (on spark plug disabled at a time, listen for drop in RPM or lack of drop).

Lastly I plan to pull the injectors and have them professionally cleaned.

[siteunseen]

That's the most fun I've gotten for $5.  I ran mine in the cabin, passenger's seat coiled up, made some markings for best performance, best MPGs, coolest motor temp.  When you get it on the interstate play with the dial every 5 or ten minutes.  It's amazing the small things you can do with the potentiometer.

[Dave WM]

I plan to shoot some video of it (colortune/vac gauge) later. There is a injector cleaning service near me that does the ultrasonic cleaning etc.... that I plan to try as well.

[Dave WM]

well well well got the colortune installed results

1&3 def look to have issues, mostly blue or no fire at idle, fine at partial throttle (purple/blue)

2,4,5,6 mostly yellow at idle.

I assume the yellow is the enriched idle to avoid starving on acceleration. The miss "seemed" to correlate with the no fire (black) missing pulses on 1&3.

when partial throttle was applied it moved into the purple zone if anything a tad rich, so I will not be using the pot as that only makes it richer. This is with a engine temps a little less than 1/2 guessing about 165f.

Next step will be to get them flow checked and cleaned.

[siteunseen]

If I remeber right it was a close call on my part to get them cleaned, new o-rings (big and small) replaced versus the price for new ones.  I'm sure i spent a lot of time getting the lowest price, coupon codes and all of that mess but i did get them cheap.  Anyway here's the thread if you want to read it. Cliff

Edited August 28, 2017 by siteunseen

[Dave WM]

IIRC pulling the injectors requires removal of the fuel rail. As such it seems in the interest of science it would be a good opportunity to hookup a fuel supply at 30 psi and do my own flow test prior to professional cleaning. I am hoping to replicate the imbalance of 1&3 to confirm the colortune as a viable approach to diagnosing flow issues, esp at idle duty cycle rates.

[Zed Head]

One big weakness of multiport iinjection EFI is that there's an assumption that all of the cylinders get the same quantity of air.  My #1 cylinder has always run lean, suggesting that it gets more air than the others.  5 and 6 tend to run rich.  Kind of seems backward since #1 is behind the air path curve and 5 and 6 are at the end.  It could be fixed with individual pulse control.  Easier to just run rich on a few.

[siteunseen]

Good ventilation and use some long wires, ho's pipe or an extinguisher.  Easy to see them spray or not.

[Dave WM]

the thing that was getting me was the higher apparent drop out rate of 1&3 as well that seems to be more related to the miss I am having. they all would black out sometimes (consistent RPM a noted random period when no fire was seen). I am guessing it was just too lean to support a combustion cycle.

[siteunseen]

2 minutes ago, Zed Head said:

One big weakness of multiport iinjection EFI is that there's an assumption that all of the cylinders get the same quantity of air.  My #1 cylinder has always run lean, suggesting that it gets more air than the others.  5 and 6 tend to run rich.  Kind of seems backward since #1 is behind the air path curve and 5 and 6 are at the end.  It could be fixed with individual pulse control.  Easier to just run rich on a few.

Zed do you have an N47 intake or a N42?  My N47 was a crossover model.  It has the EGR tunnels but know allen head plugs plus the EGR mount itself is not opened up.  Weird but it's pretty much an N42.

[Dave WM]

I have a pressure vessel that is used for flushing AC parts, I can use it with a regulated air pressure supply to supply fuel. Also have 6 injector plugs that I can wire up to activate. will use 6 separate vessels to capture flow to evaluate.

[siteunseen]

Before my potentiometer tweak my '77 would stall and then pick up up until 2,000 - 2,500 RPM.  It would spit and sputter even worse when I forced fuel on it.