V12 running RICH!

V12-Engine
1

Hi forum,

Some months ago, I bought this '78 XJ12 from a talented retired engineer, who did a complete resto on it for something to do. He built it his way, with S1,2and 3 and Euro stuff on it. He swapped in a complete HE engine and ECU and harness, and…drum roll…an E Type 4 speed. It has side exhaust, and an AJ6 enhanced ECU, programmed for no-cat use. Cool stuff, but, it’s running very rich. A set of plugs lasts a couple of start ups. I changed the oil for fear of fuel contamination.

I wish I could get this baby to run right, I want to see the results of the performance work. I did drive it 2 hours home after swapping plugs, and it was nice to shift gears, but it bogged with anything over 1/3 throttle.

Keep in mind and with a complete resto and swapping parts around through the years, anything could be wrong.

So this is what I’ve done:

  • swapped ECU’s - no difference
  • CTS seems in spec, tried jumpering the plug, and verified the harness is reaching the ECU
  • added O2 sensors, there weren’t any! and verified the harness is reaching the ECU
  • Throttle switch reads good though the range
  • both full load switches working, and the harnesses are reaching the ECU
  • cleaned power resister plug
  • vac tube from balance tube is reaching the ECU, and engine bogs out when it’s disconnected
  • Good spark, NOID light shows good pulse.

Today I had a tech come again, and he noticed during cranking, there’s no vacuum on his gauge at the intake. This could be fuel washed down the bores, or bad cam timing, or intake leak. I tried another V12 ( a friend dropped by) and the gauge showed 5-6 pounds vac

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during cranking before it fired. Coincidentally I got a smoke machine recently, and pressuized the intake with smoke. I only found a minor vac hose leak. I did a compression test one one easy cylinder, and got 175 as is, and 200 with a tablespoon of oil. So, it seems like 175 should start the engine, or create vacuum. I guess to rule out fuel wash causing the no vac, I should put oil into all cylinders and test again.

I know the engine builder, and he’s done hundreds of these engines, so I’d be surprised if the cam timing’s out. But, I’ll remove the cam covers if I must.

Any help is very appreciated,

Cheers,

Rob

2

OK, I had a similar situation on my 76 XJ12L. WHILE IDLING the left bank was running rich to the point of gas vapor coming out of the exhaust pipe.
It took me a while to finally find the problem but what was happening was the rubber elbow that is mounted just behind the butterfly valve on the intake was collapsing. This elbow is coming from the aux air valve so I had no combustion air. Now as soon as the throttle was cracked open, it would run normally usually igniting the fuel in the exhaust system with a spectacular backfire.
I found a radiator hose the same diameter as the soft rubber original and replaced it, problem solved.
I know this is a older post but just thought I would reply. Been away for a while.

3

One dumb thing you might check: A previous case with similar symptoms turned out to have been caused by some goober jacking up the car on the port side and crushing the fuel return line to the tank. The resulting flow restriction caused the rail pressure to skyrocket, which in turn causes the engine to run rich.

4

Thank you Moody and Kirby,

Moody, I checked that hose, and it’s good. For sure I’m rich in both banks. I get huge backfires and flames sometimes too. If the car didn’t have straight thru exhaust, I’m sure it would have destroyed the whole system!

I haven’t done much on the car, but will now that the weather’s coming.

Kirby, I did check that fuel pipe also, and it’s good. I blew air through it, using my mouth, and there was no resistance.

I did pull the cam covers and verified the cam timing is right. While there I found the clearances to be tight at 8-10 thou. Cause for worry?

I’m going to try using The Right Stuff gasket maker. My local tech says it’s good. Kirby I know you have your preferences for this. I hope it works.

One other thing to check is that both coils are good. One lister said he experienced rich running, and it wouldn’t rev over 3500 when the rad coil failed. That sounds like mine. When I drove it home it wouldn’t rev over 3500.

One other thing I did was ask AJ6 about the issue. I’m pleased Roger took the time to help me. He said:

Hi Rob,

If it is running rich like that then some things to check are:

  1. Fuel pressure – should be 2.5 bar with no vacuum connection to the outgoing regulator.
  2. Coolant sensor or wiring – it won’t be open circuit or the engine would just die but if there is excessive resistance that would cause richness.
  3. Engine running too cool – this can cause excessive acceleration enrichment which can bog the engine down on acceleration. Thermostats should be 82 C minimum and preferably 88 C.
  4. Faulty ignition trigger or amplifier producing too many trigger spikes. This can be seen with an oscilloscope attached to the coil negative terminal to check for any defect in the primary waveform. Sparks may still seem OK.
  5. A poor earth or power connection in the EFI wiring.

The throttle potentiometer can just be disconnected if in doubt – the engine will just be a bit flat and the idle trim won’t work.

An exhaust gas analysis is always helpful to figure out what is happening.

Best regards,

Roger Bywater.

I think suggestion #4 is the next I will try. I’ll need help though.

He also said the ECU he supplied didn’t need the O2 sensors I put in. Crap, that was a waste of time.

Thank you!!

Rob

5

This resto project is in a great progress. Any new updates about it?