Humor me .

[blues90]

This 73 T-3 is really driving me nuts. I like the car , I've owned it a long time and if I had the money I would buy something newer.

Here's the deal.

The car ran fine in 2006 . In 2009 I felt it was long over due for vacuum lines and fuel lines, injector seals . you know the basics Spark plus old Bosch W8AC . OH and I put back on the heat exchangers took off the J pipes yet with them I did secure the lower tins to the J damn pipes.

Once I was finished I ended up with not only a hunting issue but mainly this Idle that when in park and once the RPM is stable maybe 10 minutes . If I try to just bring up the idle sitting there it's fine at 1000 RPM yet a tiny bit unstable and an erratic miss at the exhaust . If I try to slowly bring up the idle it fights / drags / difficult to just raise it 100 RPM and the tach is up and down above and below say + or - 100 RPM . Once it reaches 2000 RPM is clears up then responds to an even gas peddle movement.

AS I said a few times the only thing that instantly ends this is unplugging the IAD temp sensor. If I do this the RPM shoots way up and if I bring in back to 1000 everything acts as it should. To add when it's cold and I pull the IAD temp sensor it runs worse.

Back then the only new parts were hoses and plugs , , injector seals and I went through the entire FI wiring.

In 2015 I was given the proper pressure sensor and it did help some.

Now if I place it in drive and no matter what speed it has no miss at all. I basically stay in 2nd gear so 2500 RPM max. I was on the freeway before the PS and didn't feel any issues. Even at idle last I checked put it in drive and placed my hand at the tail pipe and the erratic miss is almost none existent.

Since I replaced the plug wires , cap and rotor no change. Went over the valves twice , no change , tightened all the FI terminals , thought I felt a change.

If the new parts made no change and I used the same injectors. The only new part I have not checked for fault are the spark plugs.

I just can't see how a spark plug would cause this.

I can live with it yet feel since I've only driven at most 2500 miles since I did that work that at some point I'll regret it. If I'm starting off from a stop it goes and I feel nothing to be concerned about.

I don't have points , have pertronix way before this.

Does it seem fuel related to you? Back then I checked both temp sensors and they were where they should be. I was careful with the injectors.

[Multi69s]

It may not solve your problem, but I would change the plugs. Its cheap insurance, and I can't tell you how much time I have wasted in the past over what I assumed to be good plugs.
_________________
69 road Bug 2110
73 Squareback - 2L, T4, Automatic W/ AC
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Gone but not forgotten 72 Baja Bug 2010
My builds
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[raygreenwood]

We went through this a while back.

Some questions ai already know the answers to....but will state them for understanding.

1. Why did you replace all the vacuum lines and seals?.....because...tbey were old and cracked and probably some were leaking...right?
So if they had been in that condition for a while.....and you know that ANY vacuum peak no matter how minor.... causes a richer running condition....and the MPS was at some point adjusted to compensate for that richer condition.....it will now run poorly when you fix all the leaks.

2. What does the IAD do?......it richens and leans the fuel mixture slightly according to intake air temp. It can make ab across the board difference of between 5 and 10%. If you are already borderline rich or lean....meaning running well but just a few percentage points dich or lean short of causing an issue.....this sensor can kick it over the edge.

3. With all of this work.....djd your fuel pressure change...even by 1 psi?.....if it did.....every 1 psi difference from the factory fuel pressure setting of 28 psi.....is 3.57%....across the board...all rpm and temp ranges.

So you can see how 5-10% or so from the IAD and say 2 psi difference causing a 7% difference can be a whopping 12% to 17% difference in fuel mixture.

Also.....far too many people who see D-jet "hunting" at idle......make the assumption that bunting means LEAN.....when it will just as easily mean rich. A slightly rich nixture at idle with two wet ports in D-jet.....means a rich spike that can cause an rpm rise depending on air mix....causing a vacuum change....which causes a lean spike.
With the MPS controlling the load....either rich or lean will cause the exact hunting pattern just inverse if graphed.

Most probably you either sealed vacuum leaks you were living with and did not compensate for the loss of air in the mixture....or you have vacuum leaks that are small and intermittent.

Places to check are the runner boots, the seal on the back of the plenum, intake manifold to heads.

And.....have you adjusted the MPS?......this type of hunting is common to the MPS needing to be tweaked to match engine vacuum condition. Ray

[blues90]

[blues90]

[Clatter]

[Donnie strickland]

[raygreenwood]

Mmmmm...technically.......at idle........compared to other types of cars with different injection......yes....all four ports CAN be a little damp.....but as you will note under the heading of "fuel injection timing".....cylinders 2 and 4 the intake valves are closed when the injectors fire.

Driving these engines in lots of stop and go traffic at low rpm or....lots of idling......with technically perfect injection really only happening on one out of four cylinders.....and by this I mean.....injection with the intake valve open with intake air moving through the port to accept the entire charge of fuel so little of it settles on hot port walls.....which forms a layer of varnish over time........

This effect CAN varnish all four ports lightly. But......on cylinders 1 and 3.....the gasoline keeps the spray contact path right in front of the injectors .....very clean and almost bright. The fuel as it is swept through the open intake port.....washes the varnish out of its path.
But.....on. cylinders 2 and 4.....you get a complete baked layer of varnish because the fuel contacts the port wall constantly.

Fuel that wets to the port surface....requires about 3-5 times the airflow over the surface to pull it off of that surface because the heat from the surface in that millisecond of contact has flash evaporated out the finer constituents. The fuel has more surface area against the head than it has on its outer surface.

On high mileage cars you can see this effect....to clean ports and two dirty ports....when you pull off the runners.

Its actually mostly noticable on engines that spend a lot of time in stop and go traffic or idle a lot and is especially nasty in engines that have been poorly tuned and running rich.

The closed intake valve issue really not a running problem....as once you get up over about 1500 to 1800 rpm.....the time interval between spray and the next open valve cycle for 2 and 4 is just too short for fuel to land on and wet to the port walls.

But.....at idle....with this wet port wall issue on 2 cylinders and a very alightly,wet port wall on one of the other two cylinders because only one cylinder has a full valve open cycle during injection and one a partial valve open cycle......what you get is a build up of fuel film on the port walls.

Key here is this:.....at idle.....because some fuel lands on the port walls and sticks and is not available in the upcoming intake cycle.....that cylinder for that cycle is slightly lean.
When combusted.....this affects the output. That affects the overall vacuum signature very slightly in the next cycle.

Over several revolutions you get that fuel scavenging off the port walls and getting sucked into the intake charge.....making that cylinder alternately slightly richer.....again very slightly affecting vacuum signature on the next cycle.

What this alternate rich lean does.....in its affect on vacuum signature.....is affect the MPS....which causes the entire system to oscillate up and down in a rich lean fashion from second to second.

When this happens.....the cycle of one or two cylinders being either excessively rich or lean....causes a change in combustion that can be "felt"....as a stray pulse at the exhaust and/or an oscillation in engine steadiness at idle.....and commonly when on the rich side.....a slow hunting cycle over anywhere from 5 to 20 seconds.

Because of the two wet port cycle.....the natural running state of D-jet at idle....from the factory.....was just slightly lean if read by a tailpipe probe. This was done to prevent that excess richness that would cause excessive build up and idle oscillation.

By one important respect, factory tuning was quite sharp. It was a compromise that produced a smooth stable idle. But.....it could also have the habit by being slightly lean at idle.....to cause slightly lean mixture through some areas of part throttle.....especially in very hot climates and very cold climates.....when baseline fuel mixture adjustment from the TS 1 and TS 2.....are maxed out in one direction of the other.

A better than factory state of injection.....is just very slightly richer at the MPS to insure excellent part throttle fueling....but keeping the at idle baseline mixture under better control with TS-1 and 2 by slight ballasting. Ray

[Bobnotch]

I was of the understanding that the 72 and 73's ran better with the fuel pressure closer to 30 PSI, versus 28 PSI. I know my 71 ran better at 29 PSI. Thoughts?
_________________
Bob 65 Notch S with Sunroof
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[Donnie strickland]

[KTPhil]

I never noticed the subtle point made in the Elfrink manual posted is that he claims VW used a "modified trigger contact unit," which I take to mean trigger points. Other than adding a grease shield, I thought they were identical all years. I think he misspoke and meant the circuit in the ECU that relates to the trigger points changed the timing.

[blues90]

[blues90]

[blues90]

[raygreenwood]

[blues90]

I may be way off . It seems to me with no load where I have this issue the MPS is what see's the load if I recall. Along with that the dist advance is based on RPM and load .

Since I didn't touch the fuel pressure in 2009 which the ECU does not see. It was @ 29 PSI.

I know back 20 years ago the fuel was different so I have to leave that out.

I did get a timing light in 2009 and just checked it and it was where it is supposed to be.

Now since the spark plugs were in their boxes since 97 and those are the ones I used in 2009 . All that time I could have dropped one can't say.

Is it even remotely possible that one has an issue . I never ran across that before and it seems under load would be when a spark plug would act up like a plug wire. Both could at idle and last I checked pulling one wire at a time each one dropped the RPM by the same amount. I just pulled them at the cap . I have not checked the head temp sensor other than cold since 2009. Still it was something I changed in 2009 that caused this issue to crop up. It has not become any worse or better since then.

I can add resistance to the head temp sensor to the point it will act just like it does with the IAD sensor unplugged. And as I said when the engine is cold if I unplug IAD sensor it runs worse until it warms up 10 minutes or so. when cold I do not have that 1000- 2000 RPM struggle , It's a very short time frame to test. If you shut it down to test then the temps rise because there is no cooling air flow . wish there was a way to monitor the head sensor while running.

[raygreenwood]

[Donnie strickland]

[Donnie strickland]

Ray, it's interesting you mention the distributor. I just got mine rebuilt by Jim Adney. I sent it in because the vacuum canister was bad, but I had no idea if the distributor had ever been rebuilt. Jim said it was in dire need when he disassembled it. It's something I think anybody with an injected car should have done, because of the difference it makes in the vacuum signature, which as you point out makes such a difference in how the car runs, and on an automatic car, it makes a difference in the shifting as well.
_________________
71 Elm Green FI A/T Squareback

[blues90]

I still have the 72 distributor in my car. Even had it in the original engine.
I rebuilt the 73 AH dist . it was not worn yet needed new fiber washers . I got a set from Glenn Ring yet since the washers under the trigger can as well as the shims are a smaller OD I used the larger fiber washer yet trimmed it down to fit . They were there yet fell apart.

I was going to put the AH back in until I discovered the ground on the advance plate on the AH was shorter and would not allow the Pertronix to seat . I was going to swap the advance plate yet didn't . Either than or remove the ground on the AH and solder in one so it is like the 72 .

Both do not feel sloppy yet I have no idea what the fiber washers look like in the 72 dist. The vacuum advance died long ago and I never noticed a change it's mainly for fuel economy. The trigger point drive is tight in both.

I just don't care for points they always went so far before it would start to miss . At the time in 2009 I didn't fool with changing the dist so it was not what caused the issue I have now. Never was able to find a good 303 stamped vacuum can since. I just have it in place to hold the advance plate from free moving.

One thing I do recall is when I used to adjust the valves I used to go by the 2 turn crank method . It seems more accurate since you know you are on the low part of the cam lobe , when you go by the dist 1-2-3-4 you might be off just enough. I have an extra pulley so I found the 180 mark from TDC and marked the pulley in the car the same way . I trust the TDC mark more. Been so long I need to find the method Russ used.