Miss firing problems

[frostpinfold]

Symptoms Engine starts without a problem and car pulls well when driving. Engine idles at 1000 then as it warms idle drops to 800. So far great.
Then after about 10 minutes of running car still pulls well above 1500 rpm. However it misfires at low speed. If you remove plug leads one at a time, one will have no effect on running but not always the same lead. I assumed it was ignition, but not leads or plugs.
Why after 10 minutes?
I discovered the coil was being fed with 12 volts not through the ballast resistor. (Coil is 1.5 Ohms)   I put that right and then replaced all of the following :- Coil/distributor cap/rotor arm/electronic ignition module, plugs and spark plug leads.
No change with replacing any of the ignition parts. Could it be the Bosch injection pump? Why only at low revs? Above 1500 rpm it runs very well.
The engine was restored to as new condition about 400 miles ago by a well respected engine builder.
Any thoughts would be most welcome as I'm out of ideas.

[Cees Klumper]

Still sounds electrical in nature to me. Maybe a poor ground somewhere? I had an intermittent lack of spark some years ago that manifested itself at temperature due to a poorly crimped wire going to the distributor.

[ja17]

Check fuel pressure and volume. Make sure ignition is correct. Do the split linkage test as the engine warms. Most likely you will find a lean running condition as the engine warms. You can also leave the fuel pressure gage in place to monitor the fuel pressure as the engine warms.

[roymil]

After seeing Joe's note I wondered if any change in fuel pressure is normally expected as the engine warms up?  I do see mine drop some as it warms, starting near 11PSI and dropping sometimes as low as 7PSI.  Is a 20-30% pressure drop normal or OK as the engine heats?   I assume this is a thermal effect but I've wondered if this could also be due to additional dirt getting sucked up in the fuel filter as the engine runs longer and drops the pressure.  Then settling out after shutdown and cooling, and repeating cycle the next time the engine starts cold.   I cant think of other reasons why pressure would change but maybe others know?

From my reading of previous posts it seems that having good flow volume is the most important thing to measure @~1 liter/15seconds (I think)?   I know for sure that when I've had clogged fuel filters, flow is low and engine runs poorly, likely lean, regardless of pressure.  So agree from experience, checking fuel flow volume is really important.    -Mark

[ja17]

Your fuel pressure is too low. You will have all kinds of lean running problems after fuel pressure falls below 11psi. Pressure will vary with rpms slightly. Check to see if you have an original electric fuel pump. Next check the gas tank filter screen, the electric fuel pump filter screen, change the main fuel filter if you have not.

[Kevkeller]

Joe,
Why would the fuel pressure drop as the engine warms up?

[ja17]

As the engine warms up the RPMS lower and the voltage in the system lowers also. The electric fuel pump is a DC motor and the RPMS will increase or decrease depending on the voltage in the system.

[frostpinfold]

I have not checked the fuel pressure and flow, which I will obviously do to check if this a problem. However can anyone explain why the engine runs on less than six cylinders when the rpm drops below 2000 but above that goes very well. I have not yet taken the engine above 4000 as I only have about 400 miles since a complete rebuild of the engine. But between 2000 and 4000 the car performs very well.
I would have thought if the fuel pressure or flow were low it would affect higher rpm rather than idle, or am I  missing something.
Rob

[Pawel66]

Let it idle in a completely dark garage. Look for sparks. I am serious.

[ctaylor738]

Suggest:

Check your idle mixture with the split linkage test or a CO meter.  Check the linkage ball studs to make sure the air side isn't moving first.  Make sure that the vacuum system is delivering advance at lower rpm when you hit the gas.

[mdsalemi]

Let it idle in a completely dark garage. Look for sparks. I am serious.

Great tip!

In November 1978, while taking a drive from Pittsburgh to Rochester, NY in my beat up old 1972 Toyota Corolla, in the rain, I started getting some serious misfiring. I pulled over in a rest stop, which was not brightly lit, and popped the bonnet. Lo and behold, the engine bay was "sparking" like a Christmas tree!  Everything was kind of wet and oily, and the "inboard" spark plugs on that car exacerbated the situation. I shut it down, took a rag and wiped everything clean including the wires, as best I could. Solved the problem. No more sparking and no more misfiring.