idle problems - weird one

[jkeyzer]

I am having problems setting the idle on my 72 1.7 (doesn't everyone at one time?).

I have set the dwell and timing, replaced most easy to replace vacuum hoses, checked the injectors for equal flow, spray pattern and leaks, checked the AAR for operation, checked MPS for part #, resistance and vacuum leaks, checked the distributor retard for vacuum leaks, and checked both temp sensors for cold and hot resistance. TPS is set correctly. Compression test was good.

My problem is that I seem to be idling lean. Unscrewing the idle bleed screw all the way doesn't raise the idle above 700-800 RPM, and it is very choppy. If I turn the ECU knob all the way to the right the idle (full rich) improves somewhat but is still not ideal. If I remove the TS1 connector on the airbox the idle increases a lot and is actually pretty good. It seems like that gives it just about enough additional fuel to run well (at idle at least).

The part that confuses me is that my AAR works. I would expect that the additional air from the AAR would make the car stall (really lean), but instead it raises the idle up to 1500-2000 rpm like it should for a few minutes. Actually, it goes something like this:

Start car. Give it some gas.
Idles around 300 rpm, very rough.
10-20 seconds later, idle comes up to 1500-2000 rpm.
Idles high 1500-2000 rpm for a few minutes until starting to warm up.
Idle drops down to 500 rpm or so and is lopey, engine vibrates and exhaust note is somewhat rhythmic.
It continues like this as long as it runs.

So if the AAR just lets in more air, not more gas, why is the engine happy to run at 1500 rpm but not at 900?

I have noticed that if I disconnect the vacuum line to the distributor the idle jumps up to around 1000-1200 rpm and is smooth even with TS1 connected.

As a side note, when I have this very lumpy idle, I connected a vacuum gauge to the MPS line and the pointer vibrates wildly (10 in Hg range). Is this just because I was idling around 500 rpm? Alternatively I connected to the distributor vacuum line (disconnecting from the dizzy raised the idle to around 1200 rpm) and it was pretty stable around 16 in-Hg.

I'm not expecting anyone to fix my problem for me, but any advice or pointers (or been there done thats) would be appreciated. There are lots more things to check but that AAR and idle speed thing has me confused.

Thanks in advance to everyone who replies...
Jeff

[Dave at Pelican Parts]

First, the AAR may not add more fuel itself, but it does cause more fuel to be added by the rest of the FI when it lets more air in. More air in raises the pressure in the manifold, which is detected by the MPS, which adds fuel to compensate. No problem.

Your vacuum readings sound pretty normal.

It sounds as if your idle mixture is lean.

The thing with the distributor vacuum line (there should be two, I believe) is not unexpected. The line is most likely vacuum retard (plugs into the fitting facing back toward the distributor). When you un-retard the timing, the idle should speed up.

You can try raising your fuel pressure--make sure you use a gauge to tell you where you are right now and also to tell you where you wind up. Going from the stock 29 PSI to about 36 will probably not cause the injectors to fail, or to not spray correctly. At the least, you can use that as a diagnostic tool.

--DD

[3D914]

Dave,

Couldn't a failing retard diaphram cause the points plate to move too far, resulting in more retard, which in turn would reduce the idle?

Just a thought.

[Dave at Pelican Parts]

Hmm, sounds backwards to me. The diaphragm gets pulled on by manifold vacuum, moving the arm that moves the points plate. If the diaphragm is ruptured or leaks, then you'd get no vacuum retard rather than too much.

--DD

[3D914]

OK - maybe not failing but pulling too far. I recall one shop indicating the retard moves only 5deg, which would represent about 1/8 in movement of the arm. Mine was moving almost .25in. measured in the linear direction of the arm at the pivot pin.

Curoius!

PS: What is the maximum fuel pressure the injectors can hold (for 2.0L engines) - different than 1.7?

[Dave at Pelican Parts]

That they can hold? I don't know, but it's up there! That they have a decent spray pattern? I am told about 36 PSI. That they can hold for a long time? I am told, again, "about" 36 PSI...

You might have the wrong vacuum dashpot on there, perhaps?

--DD

[jkeyzer]

I also set fuel pressure with the engine off and it was around 27 psi, so I brought it up to 29, this had no effect that I could tell.

I wonder how stock this engine is after all these years.