348 MAF adjustment and open loop?

Looks like fun.

 

I did the EGT readings today, factory non-cat 348, the gauge connected to the L&R Thermocouples, engine idling only, with the MAF pots as set some time ago - both at 382 Ohms. The first readings are before the cold engine start (ambient temperature). The subsequent readings are at approx. 1 min. intervals, total time about 10 min. The difference L-R started at about 8C (the top temp on the gauge is the left bank) then grew to the max. of 18C when the EGT-s were around 200C. When the EGT-s reached about the maximum they will go to at idle (~265C), the difference dropped to ~10C. I guess this was due to some equalisation done when the O2 sensors came into play (closed loop). For the next test (tomorrow, from cold engine), I will turn the pot on the left (cooler) MAF to make the bank run a bit leaner and see if its EGT gets closer to the right (hotter) bank.

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I intend to do the tests step-by-step. Higher ohms should be leaner, lower ohms richer. Tomorrow, I will turn the cooler side MAF screw to leaner, say 400 ohms, and see what effect it has during open and during closed loops. With this test, I would like to establish whether to CO screw has any effect in the closed loop or not. If not, then my recorded difference of 10C at around 265C EGT-s should remain the same but I may see less difference during the warm-up open loop. The first observation I have made is that the difference of about 10% during the warm-up (open loop) dropped to just 4% when the EGT-s reached ~265C and, presumably, it was closed loop by then.

I prefer not to disconnect the O2 sensors as the engine will run pretty rich and probably have rather rough idle. Instead, I will do one test per day (no hurry) and see if I can first achieve EGT-s with minimal difference during the open loop. I have read that the difference L to R of the CO pots resistances could be as much as 100 ohms when the L & R banks are equilised using a gas analyser.

 

Unfortunately, I will not be able to continue my EGT tests for some time. Hooked-up the EGT gauge today but, when I started the engine, it immediately produced a fairly rapid sharp metallic knocking sound. Switched it off quickly, probably after 3-4 sec. Cranked the engine briefly a few times (without it starting) and did not hear any knocking. Started it one more time and, after a very brief metallic knock the engine smoothened out, no misfiring, but then I heard a few occasional faint knocks (from one bank, I think) so I switched the engine off and will not restart it until I look at the pistons with a borescope and check the compression.

Checked the belt at one bank (removed the top cover) and it looked good and tight. However, I suspect that it skipped a tooth or two on first start-up. The belt has done just some 4k miles but it is about 10 years old. So, it looks like a major and possibly replacing some valves.

 

So, from now on, you alone have to handle all MAF adjustment experiments.

Thinking back now, my engine trouble (belt skip or something else?) may have actually started some time earlier. Since about 2 years ago, I lost the easy and smooth start of the engine I had before. Instead, it would always start with initially running rough/hesitating, for two-three seconds, at very low rpm and then smoothen-out and idle very well with absolutely no strange noises (14k miles only done). Perhaps today was the next step of something that was "cooking". I will report in a new thread once I make any findings.

 

Very interesting and valuable thread!!
After a lot of careful consideration, I have decided to go with a fully programmable aftermarket ECU, back when these cars were new the Motronic was top technology, but now we have much better technology, injector technology has moved on.....and I think there is a lot of potential these engines have which can be used with some new technology thrown at them.
I will install the ECU so it is fully reversible to factory if I ever decide to sell the car.
I am going up in injector size as the puny injectors these cars have are only good for about 320hp. 250cc quad spray is what I'm using. No MAF, all on TPS.
We have 98 octane here so we'll be tuning it on that which means we'll get as much timing into it as possible...

I will document the whole thing here, including dyno runs etc etc, I'm looking forward to getting the best from my F119G........

Sorry to derail the thread, back to regular programming.

 

Thanks for the info. I think this is exactly the reason for my "stumble and gurgle" (very good description for it). A few years ago, I decided to remove the idle adjuster screws to clean them up. I remember that one was almost closed and the other 1-2 turns out. It was obvious that the PO had "readjusted" them as there was some damage to the lock nuts. I think I set them both at 2.5 turns out without using a mercury balancer and left them there as the idle sounded good/smooth. The screws are probably out too much as my idle is 1050 rpm but I just found in the manual that it should be 950 +/- 50 rpm.

I will readjust them as per the manual and use a balancer this time.

 

After I established (in the other thread) that the compression at my right bank reads 240 psi and left bank 200 psi, I now believe that the reason why my right bank EGT, with both CO screws at 383, is about 10% higher than the left bank is the higher compression of the right bank that I currently have. Probably not only the right but also the left bank compression readings appear to be somewhat on the high side and this is likely due to excessive carbon build-up on the pistons, maybe more in the right bank for some reason. I will continue the EGT tests after I, hopefully, manage to remove the carbon with "Techron Concentrate Plus" fuel additive that I have ordered and bring the compression down to a more reasonable level.

 

My findings so far regarding CO screw (open loop mixture) adjustment:

1. Due to over-skimmed right head, I have 20% higher compression in the right bank (150 left 180 right) but the engine idles very smooth and runs well with the CO pots at 383 Ohm. Right bank EGT is 10% higher than the left, I believe due to higher compression.

2. Moved the left CO pot to 400 Ohm (leaner) hoping to bring the left bank EGT a bit higher and closer to the right bank EGT; after starting (cold engine), the engine almost immediately developed detonation in the left bank meaning that the change 383 to 400 Ohm caused quite significant mixture lean-out. What helped starting the detonation was probably also quite a bit of carbon that I have on the piston tops and 1+ year old petrol.

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3. After moving the CO pot back to 383 Ohm, the detonation disappeared.

The EGT gauge which takes simultaneous EGT measurements left-right was a great tool for my diagnosing that my right bank (due to over-skimmed head) has 20% higher compression. The only good result is that both banks have equal compressions between their cylinders - 150 left, 182-183 right.

In view of the higher compression that I have in the right bank, I will not experiment further with the CO pots adjustments based on EGT until I sort out the right cylinder head.

 

I had the detonation at idle, ~1100 rpm, after I turned the left CO pot to 400 Ohm. However, with quite a bit of carbon on my pistons and with "bad gas" my engine was possibly close to detonation anyway so the change 383 to 400 Ohm probably pushed it over to the detonation side. In view of this, I have to withdraw my earlier statement that "change 383 to 400 Ohm caused quite significant mixture lean-out". I will repeat the EGT tests with fresh petrol and after I have run the engine for some time with the Techron additive.

 

Checked my idle screws today and they were both at 3 turns out (as I set them a few years ago). With this setting, my idle was at 1070, a bit too high. I turned them 1/2 turn in i.e. at 2.5 turns out and the start was immediately better with the idle speed now at 1000 rpm. Will fine tune (synchronise) them tomorrow using two vacuum dial gauges from my vintage 4-gauge carb synchroniser (hope it works, haven't used it for 30 years).

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