New hesitation at part throttle on 420R

[MV8]

Fuel, spark, timing, condition. See if the injectors are all blinking an led test lamp similarly at idle (move wiring around looking for inconsistency). If so, move the injectors around to see if the problem follows. Check for vacuum leaks around the affected cylinder intakes and injector seals with a LITTLE, short squirt of carb cleaner or starting fluid while idling, listening for an rpm change.

[Pokey]

Not sure if I posted anything at the time, but I had one cylinder running colder that I eventually traced back to the ignition signal coming from the ECU.  For me the symptoms were more binary than for you, all of a sudden I (partially) dropped one cylinder.  Granted, symptoms were more or less acute depending on driving conditions, and I don't know if the MBE ECU in these 420's fails the same way every time but perhaps you are chasing this gremlin?

 

The definitive test is to put your ECU into another car and see if the problem follows, but I got to that point in troubleshooting by using a logic probe starting at the coil and working my way back in the wiring harness until I was convinced it was the ECU signal. 

 

 

[ashyers]

"Unplugging the rear cylinder created no change in idle, however revving is even worse. I swapped coil harness and no change. Swapped coils between 2 cylinders - same behavior."

 

You disconnect the coil to cylinder #4 the idle does not change. Seems cylinder #4 isn't contributing at idle and has an issue.

 

You swap the coil harness and no changes. Either you have 2 identically bad harnesses or it's not the issue.

 

You swapped the coils between 2 cylinders... You lost me here. Did you swap the coil of #4 with another cylinder and the problem followed the coil or #4 is still not contributing any power at idle with a different coil?

 

Andy

[KnifeySpoony]

The problem did not follow the coil - it stayed with cylinder #4. 

[JohnCh]

1 hour ago, ashyers said:

You swap the coil harness and no changes. Either you have 2 identically bad harnesses or it's not the issue.

 

 

Another option is with the engine loom plug to which this harness connects.  I've seen factory harnesses before where a pin isn't fully seated which creates an intermittent issue.  Easy enough to check and rule out. 

 

 

[fastg]

If you have swapped, the plug, coil and harness around with no effect, it might be time to look at the injectors. 

 

Graham 

[KnifeySpoony]

Thanks all - I'm lazy and I hate wrenching/troubleshooting (although I did build the damned thing), so I try to gather as much information as I can and do the least amount of faffing about. Try to do the easier and/or high probability things first. I think I am going to start with continuity checks on coil wires to ECU and O2 sensor to ECU. I still think that there may be some link to open/closed loop running. I just can't see how the car is doing PRs on track but can barely make it to the track...

[KnifeySpoony]

On 8/8/2023 at 9:57 AM, MV8 said:

Fuel, spark, timing, condition. See if the injectors are all blinking an led test lamp similarly at idle (move wiring around looking for inconsistency). If so, move the injectors around to see if the problem follows. Check for vacuum leaks around the affected cylinder intakes and injector seals with a LITTLE, short squirt of carb cleaner or starting fluid while idling, listening for an rpm change.

Do you mean using a noid light (I literally just learned that these exist)? Any of these cheap sets available on Amazon will do I presume - where are there multiple lights in a set - different connectors?

[MV8]

Essentially. There are many different injector connector designs. I use a traditional incandescent style test light probe with a clip lead to back probe the connector but a snap-on "noid light" would be easier to use. Look for a set that cover fords if you have oem type injectors.

 

It sounds to me like you've pretty well covered ignition. Idle and transition from idle is more critical than midrange where there is a lot of airflow, inertia and rpm, so the problem won't be as noticeable once moving but still exist. Injectors have a tiny "last chance" screen that is easily clogged. An O2 sensor typically is not a tuning input at idle.

 

On a side note, one thing I like about the old throttle body injection systems (TBI) that filled the gap between carbs and multiport was the ability to see the injectors spray pattern just by looking.

[KnifeySpoony]

Seen some threads on blatchat. Some people with similarish symptoms from throttle body imbalance. My throttle bodies are visually balanced at idle and wot, but haven't tested manometer. Worth doing? 

[KnifeySpoony]

I reached out to Bruce Beachman for some advice - we went through all the usual suspects, but he was sure to impress upon me the importance of TPS idle voltage. Which, of course, was the first thing I checked, but I didn't realize that being off just a few hundredths of a volt could induce such behavior. Also the replacement TPS I put in had idle voltage that wandered a bit and had excessive hysteresis. I swapped back to the OEM TPS and at 4.62V seems to be OK again. Will give another report after proper test drive.

[sltous]

Adding some other context for problem solving in the future.  I provided the problematic throttle position sensor and when I look back through my notes this sensor does not have a return spring.

 

My theory based on this lack of a return spring is that at low/idle throttle positions the positioning slop of the sensor permits large measured changes in position from the vibrations of the engine compared to the actual change of the throttle so in essence the idle voltage is wandering because the engine is idling.  If the sensor has a return spring the wandering of the idle voltage should be much less because there is a resultant force maintaining the sensor position against the actual position of the throttle.  The position of the sensor when driving at speed should be relatively quite accurate because the throttle has moved a large distance compared to the slop in the sensor position so the theory somewhat matches the described behavior where you are setting PRs but can't drive home in traffic.

[KnifeySpoony]

Did another slightly longer test drive. Pulling smoothly. However, after car got warm, the idle speed crept up to about 1800rpm. Checked the TPS idle voltage and it had predictably crept down to 4.55. I have ordered two more genuine Ford TPS units. Hopefully I can get one that is reliable...

[sf4018]

Sure it's not the throttle cable not returning all the way? I had to add a spring to the pedal to make sure it returned, the spring on the baffle alone wasn't strong enough.

[KnifeySpoony]

Yes I can hear the barrels slamming onto the idle stop without limitation. I suppose the idle stop could have moved, but I think that's unlikely given the fact that the TPS voltage had shifted the other way previously.

 

[KnifeySpoony]

Gonna start a new thread on drifting TPS...