Hot Idle Problems Help

[jackmech]

Sorry for the long post, but I wanted to be clear.

I'm having problems with hot idle. My car is a 73 1.7. Same old story, it runs fine before it warms up, but then it won't idle. The weird part is that if I pull the plugs on the injectors one at a time, pulling 1 or 3 will kill the engine before I can plug it back in, but pulling 2 or 4 will just lower the idle to almost dead levels. And even then, 4 seems to have less of an effect than 2.

1. Trigger Points
My trigger points seem to be working okay. I checked them with my oscilloscope and got a nice square wave pattern on both contacts.

2. Air Temp Sensor
I usually run with the air temp sensor unplugged, since plugging it back in gives worse performance while the engine is warming up.

3. Head Temp Sensor
Replaced with a new unit from Napa Auto Parts. Measures at 70~80 ohms hot. Does this seem low?

4. Hoses
I have unhooked the vacuum retard and the decel valve. Both are plugged off. I wouldn't expect this type of behavior from leaky hoses, because it is my understanding that leaking hoses cause a high idle and not a lean/rich condition.

5. Fuel Pressure
Checked to be at about 28 psi. I was reading something about kinked lines causing problems. Could a kinked line give correct pressure, while lowering delivery rate? Or would you notice a pressure drop if you drove with your guage connected so you could see it? If there's a problem, could fuel pressure change under load, and look fine at idle?

6. Ground Connections
I spent about 1.5 hours cleaning the engine ground connections about 3 weeks ago, so I hope there isn't a problem here. Has anyone used dielectric grease on ground connections to stop corrotion and give a better ground?

7. ECU
Part number: 0 280 000 037
Also, 022 906 021E

8. MPS
Calibrated to '73 1.7 standards by our own Jeff Bowlsby. Tested to be in acceptable condition.

9. Valve Adjustment
I adjusted the valves last week. All the valves measured to be in very good adjustment before I changed anything and the car ran about the same before and after.

10. TPS
Recently replaced witha new unit from Pelican. Seems to be adjusted correctly, idle circuit is on when throttle closed and breaks when openening the throttle only a tiny bit.

Well, that's all I can think of right now. Any ideas?

[Dave at Pelican Parts]

The seals around the injectors can cause odd problems when they leak air. I'd think about just replacing them, as they're cheap.

You might check the flow of the injectors and look at the spray pattern.

--DD

[jackmech]

I realized I messed up my description. I'm fixing it now.

[hardflex]

have you checked the compression in your cylinders?

[pbanders]

Originally posted by jackmech
Sorry for the long post, but I wanted to be clear.

I'm having problems with hot idle. My car is a 73 1.7. Same old story, it runs fine before it warms up, but then it won't idle. The weird part is that if I pull the plugs on the injectors one at a time, pulling 1 or 3 will kill the engine before I can plug it back in, but pulling 2 or 4 will just lower the idle to almost dead levels. And even then, 4 seems to have less of an effect than 2.

1. Trigger Points
My trigger points seem to be working okay. I checked them with my oscilloscope and got a nice square wave pattern on both contacts.

What are the contact resistances as measured at the ECU plug? Both should be very low, check my web page for values. IIRC, under 0.2 ohms. How old are they? If you don't know, or they're well over 50K miles, replace them.

2. Air Temp Sensor
I usually run with the air temp sensor unplugged, since plugging it back in gives worse performance while the engine is warming up.

Not a good sign. When things are right with the motor, you do not need to disconnect TS1.

3. Head Temp Sensor
Replaced with a new unit from Napa Auto Parts. Measures at 70~80 ohms hot. Does this seem low?

Sounds correct. What is the overnight cold value?

4. Hoses
I have unhooked the vacuum retard and the decel valve. Both are plugged off. I wouldn't expect this type of behavior from leaky hoses, because it is my understanding that leaking hoses cause a high idle and not a lean/rich condition.

Here's another indication that something is wrong. Disconnecting the vacuum retard should require you to screw the idle bleed screw in very tight to get the idle to 950-1000 rpm. On my car, I'm pretty sure w/o vacuum retard I could not get the idle down to this level, even with the screw in all the way. Check your timing, I'm betting it's retarded.

5. Fuel Pressure
Checked to be at about 28 psi. I was reading something about kinked lines causing problems. Could a kinked line give correct pressure, while lowering delivery rate? Or would you notice a pressure drop if you drove with your guage connected so you could see it? If there's a problem, could fuel pressure change under load, and look fine at idle?

Delivery problems become apparent under heavy load, not idle. You're probably ok.

After the mechanicals and the ignition system have been checked out, as well as basic FI system condition, component matching, fuel pressure, the next step with an idle problem is to measure the idle CO level with the engine fully warmed up. You're generally (varies by engine, see the manuals) shooting for about 2.5% or so (3% works for me). What you're looking for here is if you are WAY off - I've seen cars as rich as 8% at idle, and it can be very lean, too - like 1% or so. Either extreme will result in poor idle performance. Usually the "bogging idle when hot" is due to an overly rich mixture.

[jackmech]

Ack! $100 for trigger points.

[chrisreale]

Yea, no ****! I just tossed a set of trigger points that lasted about 3000 miles

[hardflex]

the is the cylinders which are not up to snuff, 2 and 4.

if it's trigger points, it would be 1 & 4 or 2 &3.

if it were grounds, it would be 1 & 2 or 3 & 4

that says to me your problem is at the cylinder level, either bad injector patterns as Dave suggested, or leaking valves. I assume that the spark is good since no spark would cause the change at all when you disconnect the injector plug.

check the compression first, then test your injectors if the compression is good.

[Dave at Pelican Parts]

Or a cam lobe... Cam lobes are shared by cylinders on both sides of the engine--#2 uses the same lobes as #4. When you have problems in front/rear pairs, that is one possible indication of a bad cam.

A compression check is definitely in order!

--DD