A bank Choked plugs Pre HE

Hi Guys . I am just recommissioning my 76 Pre HE and have just had all the injectors back from a professional refurbishment. Slightly surprised that it didn’t fire up on 12 straight away , so pulled the plugs to find all are good burn on B bank ( front right from drivers seat) but all are fouled on the A bank. Seems a clear problem for me to solve, but not sure what would cause that. Unlikely , I would have thought to be ignition as I am assuming that A and B bank share same ignition. Therefore I was wondering whether the enrichment device was overfueling the A bank. Anyway, I am sure someone with the knowledge I need will be along in a minute. At least its not just one or two plugs on different banks. Whilst I await your very valued advice, I shall pour myself a beer and start reading the manual ! Thanks in advance Andrew

I do not claim expertise in this engine nor it’s ignition system.

But, I do know that one, the Marelli system is two six cylinder systems. One atop the other within the distributor!

Check for spark on one or more of the cylinders on the rich bank. I suspect none to be found.

Why? The “fiddle” factor. connection broken on a wire during the injector swap…

Or pure coincidence.

Fix one issue and another pops up!!

Fiddle example, non auto related. It is really hot here. So, I punched the button to start a small fan and circulate AC. No go ???

I moved it to the kitchen table to mess with it and see why? Tried again. Zoooom!!! Put it back on place. Blew nicely all afternoon and into the night. Hope it does later when I go up front for the afternoon’s hibernation. 100 F + yesterday. More of the same expected today!!

I think I’ll defer shop time…

You’re right, it’s not ignition. There are two general categories that it might be. First is an ECU fault, but even this is unlikely on a pre-H.E. as each circuit within the ECU fires three cylinders grouped together in groups of six, three on each bank. The other, more likely scenario, is that your fuel pressure regulator has failed on that bank, causing the fuel pressure to be too high on that bank only.

Most likely on this year car is the idle air hose which is a 90 degree hose under the throttle body which feeds that side of the engine air for idle. It gets soft with fuel, heat and age and will collapse under vacuum causing a overly rich mixture on the left bank. Same holds true for the right side. Good idea to change both with proper reinforced fuel compatable hose as it holds full engine vacuum.

Thank you Guys. Two great leads here for me to check tomorrow. It seems to me over enrichment rather than ignition, which could be either of your suggestions. Cannot recall the 90 degree air feed hose under the throttle body at the moment. Certainly a 90 degree from the air box to the manifold from memory. Will take a closer look tomorrow. Need to try to understand how the enrichment device works, but easy to pinch off the feed pipe which will give me a chance. IF it has failed open then it will be feeding a constant flow of fuel into the inlet manifold and choking the plugs. Sounds a strong contender.

Also your first fuel pressure regulator on the A side could be blown, which would cause fuel to leak through the vacuum hose into the intake.

Great thanks Greg. Will be getting on this today so will report back. Sadly both these parts, the cold start and the fuel regulators are pretty much unobtanium over here in the UK, which leaves you with take offs if you can find any of those as well.

This is a pre-H.E. I don’t believe the FPR’s have a vacuum hose.

The early fuel pressure regulators work together as it is a common fuel pressure system and if one fails, the other works fine unless it sticks open which will cause low pressure and weak mixture. There are no vacuum lines to the regulator. If only one bank is rich, you need to investigate what only effects that side. The injectors are in 4 groups of three and they are staggered. Only other item that might be causing a problem on one bank is the cold start injector. Easy to isolate by just pinching off the fuel line going to it. This system is adapted from the early Volkswagen so not really complicated.

I was hoping this was going to be easy! It is not proving so. To add more to the story. This is a low mileage one previous owner, well maintained non messed around with very early Pre HE. Number 572 off the line from memory. It was laid up about 1990 and I have been working through a recommission. This has been a new trigger board ( fitted by the dealer who sourced the car and sold to me ) and I have had the injectors overhauled by the UK expert. Also fitted all new fuel lines ( rubber ) and a new fuel tank as well as fuel pump , filter and cleaned the sump tank. Its had all new spark plugs NGK BP5ES. Basically its still running on about 10 cylinders. I cleaned all the plugs and pinched off the cold start injectors and it made no difference. I reckon the A bank of plugs are less sooty than the B bank because the A bank are not really seeing any fuel . If you listen to both exhaust pipes its not on a smooth six on either ( they are separate exhaust on each side ) . I just pulled three front injector leads off the A bank and the it made no difference at all to the running. The trouble with these old injector cars with electronic ignition is they are seriously hard to diagnose without a modern day diagnostic machine. It could be so many things, ECU, amplifier, manifold pressure sensor, trigger board, cold start, pressure regulator, fuel pressure, air leaks, broken wiring injector wiring loom…and so on and so on Anyway, as I am beginning to regret my purchase I would be grateful of any further advice and ideas . Here is a picture of the trouble maker in question !

Love the color!

I got to the end of my rope back in January, and it ended up being a bad ECU. Since used pre-HE ECU’s are quite cheap, I’d add that to your list.

Not sure how the Pre-HE ECU’s work, but for me, disconnecting vacuum or CTS to my HE 16CU ECU made no difference in idle, which made me question ECU.

Agree with Greg, it could be a bad ECU. It could also be a fault in a wiring loom, though. You should check the circuits from the ECU to the injectors and from the injectors to their power source.