F355 Head Scratcher

I have a 97 F355 with a stick shift in my garage that is proving to be a head scratcher for me. I would like some fresh perspective on this as to what to do next.

Symptoms: when car is warmed up, it has severe hesitation and jerkiness under any load, uphill or just applying throttle, and the hesitation is to the point of the car undriveable. But, it has no CEL, ever. Throughout the continuous trouble shooting, the basic clues come down to a few things:

Clue 1: Bank 1 long term fuel trim (LTFT) is 11% and Bank 2 LTFT is -10% (so one bank is rich while the other is running lean).
Clue 2: In closed loop, the rear O2 sensor signals are not stable at a single value (like 200mV or 100mV) but they vary from 100mV (lean) to 800mV (rich) but not necessarily following the front O2 sensors. The car does have HiFlow cat converters (but I have seen these working well before on other cars and the rear O2 sensors do stay solid at a single value and not varying so much). Still, no CEL at all.

So far, this is the list of things that has been done:

I. Compression test: all eight cylinders 195 psi or higher

II. Fuel System:
1. Fuel pressure at the fuel filter is 55 psi. Unable to measure at the injection rails since there are no Schreider valves.
2. Fuel pressure regulators both changed to new.
3. Fuel injectors removed and sent out for cleaning. One had slight leak down but was brought back to specs.
4. Found one leaky seal (out of 4) on bank 2 injector rail and corrected that.
5. Removed fuel pump and found rubber pieces disintegrating inside the tank, The rubber pieces were replaced.
6. Changed to a new fuel filter
7. No fuel leaks found anywhere, including the vacuum lines at the fuel regulators
8. Hooked up an oscilloscope to all eight injector signals and found the waveforms to be correct and 0V to 13V as expected (so they are not missing, or shorted, or grounded somewhere inside the harness)

III. Ignition System:
1. New spark plugs,
2. Removed the ignition wires from the skinny channels and let the wires hang loose without being pinched
3. Changed both ignition coils to new coils
4. Hooked up an inductive timing light to each spark plug wires and observed that the spark signals are there, and at the right frequency. There seems to be two wires on bank 1 that are double the frequency as if there is a wasted spark.

IV. Others
1. Smoke tested the intake and found no vacuum leaks at the intake
2. Smoke tested the exhaust and found only minor leakage at the donuts between header and cat converter on bank 1.
3. While car is running, there is a clear "Pht pht noise" coming out of this donut interface. The donut gasket is quite old and falling apart and might be causing the front O2 sensor to read too much oxygen, and caused the ECU to richen the mixture.
4. Headers are non-Ferrari and without any heat shields. It is quite clear that there are no leaks or holes in the exhaust headers.
5. Removed and tested the secondary pump check valves on both banks. The one on bank 1 was clearly broken and allowed air to flow both directions. This was replaced.

At the present: with all the work above, the car is now drivable but I can still feel a little bit of hesitation / jerkiness under heavy load, and I can hear faint popping from the exhaust when the hesitation happens. But, I can power through it with a heavy throttle unlike before when it will not respond to heavy throttle.

I am running out of cheap fixes and diagnosis. Anything more at this point will get expensive fast. Engine removal to check cam timing and tensioners, inspection of wiring harnesses cannot be all done with engine in. Inspection of valve lifters cannot be done with engine in. So I am not quite committed to that work yet.

What other "cheap and simple" diagnosis can be done before the engine out ?

 

Aren't all the spark plugs supposed to be wasted spark?

How old is your crank sensor?

Do you have something you can wrap around the join as a temporary measure for evaluation?

 

Crank sensor is old but I did check the wiring to be good. Although, this is a cheap fix to just change it out for one of the non OEM equivalent.
Wrapping something around the gasket is a waste of time. I would just replace it with a new one. While that side of the engine can be at least explained, bank 2 running too rich has no explanation... yet. And I still don't know why the rear O2 sensor signals continue to switch back high and low and not stay in one steady state.

 

I don't understand your LTFTs. Doesn't the negative LTFT value indicate the left bank (bank 2) is running rich? If you had a leak, shouldn't it show lean?

I thought the O2 readings were supposed to hunt about an average value. Here are two of my favourite videos on fuel trims...





Focusing on the hesitation...

When you say "power through it", do you mean you give it more throttle or is the throttle already at max and you wait for the engine to catch up?

Could there be a dead (-ish) spot in your throttle potentiometer? At WOT, the ECU is supposed to add extra fuel. I don't know if the ECU monitors the throttle potentiometer at all points (during normal operation). Does your OBD2 reader give a readout of the potentiometer position? I can put an ohmmeter across mine for comparison if you like.

 

LTFT is positive, meaning the computer is thinking that Bank 1 is lean and it keeps adding fuel.
LTFT is negative, meaning the computer thinks Bank 2 is running rich and it wants to remove fuel.

O2 sensor BEFORE the cat converter switch switch between 800mV (rich) to 100mV (lean) but O2 sensor AFTER the cat should remain steady at 200-300 mV. This is how the computer monitors the efficiency of the cats. I have seen hyperflow cats behave correctly (Rear O2 sensor steady at 100-200mV) so this is abnormal and I don't know why yet. No CEL.

Power through it means I smash the throttle to the floor and the car overcomes the hesitation.

And yes, I should measure the throttle position sensor. If you can give the resistance reading at closed throttle, 1/2 throttle, and full throttle, that would be a clue.

 

On a 348 TPS at closed its as close to 0 as you can get, 355 is likely the same> You can measure yours and see if they are different to begin with. The number is irrelevant in that if the are very different that's the issue, replace them both, if they are close to the same its likely not the issue as they both would have to fail at the same time with the same values. Sounds like Air/Fuel issue.

I am a replace everything guy since everything is old and will eventually fail and I hate prob ems every few months. I would go in this order with vacuum being at the top of the list, its likely one of those (probably vacuum), if not the car will run better more reliably anyhow and those will then not become the next problem 200 miles from home.

• Vacuum leak (smoke test maybe)
• Clean injectors (they need it anyhow)
• TPS (Search Bosch number you can get them for < 100)
  
• FPR (search xref you can get them for $75)
  
• Crank Sensor (I used OEM)
• Phase sensor (I used OEM, I got it for $85 from Tom Vail)
• Ignition module (search xref you can get them for < 100)
• Coils (search xref you can get them < $100)
• Fuel Pumps (this one sucks to do)

Best of luck!

 

That's why I'm confused. If there is an air leak, doesn't that mean you're getting too much air at the O2 sensor? Isn't that identified as "lean"? Isn't the MAF value determining initial fuel flow and if the O2 sensors are not getting enough sensed "fuel" in the air/fuel ratio, that means the car is running lean?

I'll see what I can do. I'll be working alone, so I will probably have to move the throttle cable at the engine. I'm not sure how easy it is to judge 50% or if my cable and pedal is adjusted properly.

 

What about cat ecu'?

 

Not much luck with the TPS readings. I couldn't find any suitable sized test sockets to put on the TPS connector pins. The best I could do was hold the ohmmeter probes on the outer pins and take two measurements.

No throttle: 2.65 K ohms
Full throttle: 1.29 K ohms.
I could only guess the midpoint and that produced 1.8 K ohms (which is quite far from an average of 1.97 K ohms). I couldn't tell if it was smooth or not.

It was a lot harder than I thought it would be. I had to lift up the coolant tank to get to the throttle quadrant.

 

I'm liking the crank sensor idea. Would fix the exhaust leak but doubt that is your problem.

What is o2 MV under load.

 

I could be wrong but I don’t think it even looks at that once in closed loop unless there is a problem

 

WOT and engine rpms above 4000 causes the engine to go into open loop operation (see WSM C31)
Also see C35 in the WSM "Full power enrichment" (also mentions spark advance)

 

These are left to be done, or inspected and measured:
TPS (Search Bosch number you can get them for < 100) NOT DONE.
Crank Sensor (I used OEM) To be done, i have the sensor
Phase sensor (I used OEM, I got it for $85 from Tom Vail) (To be replaced, it is cracked across the male connector)

 

Have all green ones and I did not think this can be a problem.

 

I did find that long term fuel trims of +/-10% is acceptable. This car is just over the borderline.
An additional data point / clue: the oil pressure is much too high. I am not yet sure why, it could be oil grade, or plugged oil passages. To be investigated.
From above, WOT and higher RPMs run in open loop and full enrichment. I did observe that after a full WOT run on my OBD2 scanner. Plus the ECU must look at the throttle position because it has to know when to enable the bypass valve.

 

I did inspect the wiring on the crank sensor and no issues were found. I might just change it anyway, it's cheap and quick.
I cannot read the rear O2 sensor signals while driving it. It's too hard to see.

 

Here are the couple issues I really like to sort out and understand.

Why is one bank too rich and the other bank too lean?
Why is the rear O2 sensor signals (at idle) swinging between lean and rich and not stay stable?

 

My comment won't really help you but it's interesting to read through This. Thank God I have a reputable local Ferrari mechanic that is very reasonable I can't imagine trying to troubleshoot this

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I think it’s lack of fuel to the point of engine starvation
My. Guess is fuel pressure.
Check the vacuum lines going to the pressure regulators.
Especially if it revs good under no load
You all ready suspected that right away but the problem is you have no real way to check(crappy design)
Maybe a broken or cracked vacuume line
I know you said you found no fuel leaks including vacuumed lines but that was my guess

 

I think as I said it looks at closed and wot only

 

There is nothing Ferrari specific about this, it happens to cars all the time, google "car stumbling under load". A shop will start replacing parts with used parts known good and after 2 weeks give you a labor bill that would easily pay for DIY on all the new parts as they cycled through them all trying to find the bad part. I'd probably replace the O2 sensors just for good measure first. If they are not bad now they will be one day. Could be a combo of things too like MAF going bad, 02 going bad, weak FPR or Spark perhaps. Replacing one thing may not fix it. Ohh check grounds.

 

You said the fuel pump had rubber disintegrating everywhere....i had an f250 i was chasing hesitation on forever i replaced everything to find out that bits of the fuel pickup would get partially clogged under heavy load....i had to clean the tank and lines out thoroughly. Just a thought

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Just because it makes fuel pressure doesn't mean it's consistent across the rev range. Starvation I believe. I know when I did my fuel pump and rubber mount I replaced the fuel filter a few times. The car just got better and better. Any obstruction in the system could lean a cylinder randomly. If you can I would rig up a fuel pressure gauge and have someone view it whilst driving.

 

Also make sure the MAF is fitted/mounted nice and firm in its clamps. Spray with a little soapy water whilst running. Leaks on the eng side are not great at all.

 

I had similar problem. Tried everything. Last thing was disconnect the cat ecu and problem solved. Easy to disconnect and just try it.