UK RHD M2.7 348 Running like a B***h at low revs

After experiencing problems with rough running lately whilst driving around town I’ve embarked on working my way through fault diagnosis, but would very much appreciate comments that others here could add to steer me in the right direction.

Details of my vehicle are:
1991 UK RHD 348TS M2.7 (rarely used I hasten to add, which I’m aware may well relate to the problem I am experiencing)

My first experience of something not running correctly was during a drive to my annual MOT (safety test). The car stalled a couple of times at junctions, showed an erratic idle; sometimes rising well above normal (I checked for carpet interfering with the throttle and for throttle cable adjustment, both ok). Surprisingly for how poorly the car ran, it actually passed its MOT test. Emissions were within the prescribed acceptance limits, although the values were higher than anticipated for a cat equipped 348.

Since the MOT run I have hardly used the car for fear of damaging something. When I have driven the car it has repeatedly run very roughly anytime revs are under approx 2.5K. As soon as the revs are above that region the car runs fine. The car also starts first time on the key and will sit happily at idle, although I wouldn’t say it was a particularly smooth idle.

With a 19yr old car and having read lots of online ‘stuff’ my thoughts turned toward O2 sensors, but when I drive with those disconnected, i.e. running open loop the rough running problem is still apparent and to just the same extent. On a whim and as they were very cheap I also changed out the ignition modules but no change to performance. I have a pair of new coil packs but haven’t tried them as I’m thinking its not so likely to be the coils. Plugs all look absolutely the colour that they should be and all the lead resistances check out correctly. MAF wires look to be very clean.

Other work I have done is to plod methodically through the pin to pin checks on the diagnostic sheet I have available (is sheet 13 the final iteration for my model/year?). I have got down to the part where I need a break out box (something I am well into the process of making) but up to that section the clues are pointing to, wait for it, drum roll – Earth/ground issues.

The only two areas showing out of limits so far (up to point of needing break-out box capability test 23 onwards) are as follows:

Test 1 – Ground to ECU
With positive meter terminal on ECU harness pin 19 and negative meter terminal on vehicle (fixed ground) as required, I am getting ‘minus 9.8 ohms’, rather than the required 0.2 ohms max (If I swap the leads around I get 10.2 ohms).

I will check the battery negative ‘quick release’ connector (mine still has this fitted), it may be simply this that’s causing the issues?

Test 18 – Check for ground of Oxygen sensor
With positive meter terminal on ECU harness pin 10 and negative meter terminal on ECU harness pin 19 as required, I am getting in the region of 20 ohms rather than the required 0.2 ohms max.

Any pointers from the ‘Brotherhood’ would be well appreciated

Best Regards
Dave

 

Hey thanks for that, I'll add that to the things to eliminate moving forward.

Car only has 5K miles on it (had 2.5K on it when I bought it and I hardly ever get to use it, I'm afraid much of my time goes to house and young kids), nonetheless it still could be stale fuel having gummed something up, including injectors. It isn't stood for long periods without any use, but when used it isn't usually for many miles.

Correction to my opening thread, test 18 Oxygen sensor ground I get 2 ohms, not 20 as I said above (limit is 0.2 ohms)

 

Thanks No Doubt,

I checked the MAF's out of the car a couple of months ago and have to say visually the wires looked absolutely factory clean, but will do the MAF spray in any case just to absolutely eliminate.

I also tried the ECU reset process but it seemed to make no difference, before/after the reset I experienced the same issue.

The plan now forming in my mind is:

1. Clean battery negative break connector
2. Clean battery negative to engine earth strap point
3. Finish running through Ferrari workshop manual diagnostic sheet (finish making break out box this weekend to enable the second part of this activity)
4 Remove MAF's again and spray with cleaner (that is if I can easily get such a thing in UK automotive stores?)
5. Nurse the vehicle down to the local garage and fill tank with new fuel (I'm not able to get to the car whilst typing this but I vaguelly recall the sticker in fuel fill lid might say '95 RON', if it does do others think 98 RON be ok?)
6. Should I then add a can of injector cleaner to the tank just for kicks?
7. Reset both ECU's again

Take for long drive and report back (I'll hold off on adjusting the idle air screws at this stage and just see what happens with the above).

Should I run open loop for the time being, to keep the Lamba sensors out of the equation? My thinking here being that, as long as all else was well the car should be pretty well driveable in open loop.

 

Thanks Ricardo, that may well be the case, we've had a particularly damp out season. I don't have an airline so blowing the fuel rails will mean taking it somewhere (fills me with dread based on previous experience, pretty well everytime I've ever trusted vehicles to the pro's something has been marked when I've taken them back).

 

Thats correct, I just disconnected them to see if the engine ran any better running in open loop, it was no different so I think the O2's are probably not the issue.

 

Thanks Chris. I just have paranoia about a dealer getting anywhere near it having got it so nice (well alun of alsa automotive thats is!). I may end up having to bite the bullet if all else fails!

 

Having made my break out box I have now completed my checks on the 1-4 ECU/Loom/systems.

Test that showed values Out of limits were:

Test 1. Pin 19 to fixed vehicle ground. Should get less than 0.2 ohms, Actual = 7~8 ohms. I also get 7~8 ohms on all the other points that relate back to vehicle ground '150a' on the Ferrari wiring diagram....could anyone tell me where this earth point is physically located?

Test 18. Ground of Oxygen sensor. Should get less than 0.2 ohms, Actual = 2.1 ohms. So a factor of 10 high! I suspect this may relate back in some way to the poor ground above.

Test 19. Throttle position sensor. At rest I get correct 1kohm, but at full throttle pedal I only get 2.24kohm rather than the required 2.5 to 2.8kohm. This is something I will adjust but I don't believe relates to my poor running issue.

Test 25. speed/TDC sensor. I get the good strong oscilloscope trace 'shape' as per the fig 2 in the manual, but its min to max voltage spread is only 18v rather than 25v as seen on the fig 2. The 'missing tooth' signature is also only showing voltage spread of 23.27v rather than the required 35v shown in Fig 2. So does this mean the sensor is getting tired? Also, out of interest, for this test what does the workshop manual 'acceptable range 7 to 14v' refer to?

Test 30. Lambda signal. On the oscilloscope I get a spread of 59mV and full repeat wavelength per 7.3microsecs. Out of interest using a DVM shows swinging between 0.62 to 0.35v.

Test 33. Idle speed actuator (open circuit). should be 13.5 ohm, actual is 15 ohm.

Test 34. Idle speed actuator (closed circuit). should be 13.5 ohm, actual is 13.8 ohm.

All other tests in the Ferrari workshop manual diagnostic sheet are in limits.

Now I will check the bank 5-8 ECU/Loom to see if it shows similar.

 

Eric, I will add an additional earth to the O2's if I cant get to the basic issue. Just going to test the 5-8 side first to see where those measurements sit.

I really want to trace that bad vehicle to ECU earth, particularly that position 150a on the wiring diagram.

BTW where are you in SD? I went to La Jolla area a few times on business back a couple of years ago, PF Changs being a particular favourite!

 

Finnerty,

It's the term for a Female dog, I tried to print it in full for you but alas it got blocked.

Interesting how some very commonly used words with non offensive meanings get blocked from these sites due to their side/slang meaning. Like a b*st*rd grade file, I'm sure if I tried to type that in full it would also get blocked.

 

Back on topic, see point 150a in Word doc attached, anyone got an idea where it is physically on the car?
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