[mikelr]

More then likely the bracket on the throttle pedal that the cable attaches to is bent and you are not getting true full throttle when you put the pedal down to the mat.
Common issue on the V8, the convoluted (in my opinion anyway) throttle linkage puts considerable strain on the connection point @ the pedal bending it over time.
Check this before getting crazy chasing ecu gremlins...

 

[mikelr]

There is a "rough road sensor" that is located at the left rear of the engine compartment bolted to the frame that records vertical movement and will make the ecu disregard knock counts that coincide with these vibrations. It is possible that this has failed.
Would be a good idea to check your fuel pressure @ idle and under load. It is common for the hose line connection in the tank from the main pump to rupture and limit your fuel pressure. This would cause a slight lean condition first which would cause knock/ping and a subsequent pull back on the timing/power loss. There may be no codes generated right away as it takes time for the long term fuel trims to go far enough out.

 

[mikelr]

Hi guys,

I did a little data logging during a couple test pulls and wanted to get some insight from the team. I'll post a graph with some data after I re-take it with fewer channels captured to improve the sampling rate.

All the mundane stuff is fine (throttle position sensor reads 100%, temperatures are good, etc). Also, the mixture is fine and the O2 sensor readings indicate no hint of fuel starvation and at wide open throttle and full boost the O2 sensor is reading a little rich which I'm sure is intentional to protect the engine. The recorded boost is exactly where it should be.

However, the timing advance does indeed look pretty strange. At low load and partial throttle leading up to the pull I have about 28 degrees. When I put the hammer down the advance is retarded and drops back to 16 degrees by the time the RPM's are at 3000 and boost is building. When at 5000 RPM and full boost the timing advance is down to only 9 degrees. After a gear change and hammer down again the advance comes down to 7.5 degrees at 5500 RPM and full boost.

With such retarded timing at high RPMs it's no wonder I have no acceleration at speed.

So now for a couple questions -- what kind of timing advance should I be expecting at reasonable engine RPM's? I'd expect far more than I'm seeing, but I'm not sure what is typical since I don't have a baseline that recorded this on my car.

What are likely culprits for my ECU retarding the timing so badly? The knock sensor seems like a primary input the ECU uses to retard timing, so is it susceptible to failing in a way that makes it falsely trigger?

Knut

A N/A motor would be pulling 30+ degrees of timing at the higher end of the rpm range, boosted would usually drop one degree per pound of boost (just a rough number) so you should be in the mid twenties under load at those rpms.
Did you record any knock counts during your logging? The knock sensor can be a prime suspect to pulling ignition but intake air temps can be just as important. Do some more data logging focusing on the knock counts and IAT sensors in relation to rpm/boost/ign advance.

 

[mikelr]

Hi guys,

I did a little data logging during a couple test pulls and wanted to get some insight from the team. I'll post a graph with some data after I re-take it with fewer channels captured to improve the sampling rate.

All the mundane stuff is fine (throttle position sensor reads 100%, temperatures are good, etc). Also, the mixture is fine and the O2 sensor readings indicate no hint of fuel starvation and at wide open throttle and full boost the O2 sensor is reading a little rich which I'm sure is intentional to protect the engine. The recorded boost is exactly where it should be.

However, the timing advance does indeed look pretty strange. At low load and partial throttle leading up to the pull I have about 28 degrees. When I put the hammer down the advance is retarded and drops back to 16 degrees by the time the RPM's are at 3000 and boost is building. When at 5000 RPM and full boost the timing advance is down to only 9 degrees. After a gear change and hammer down again the advance comes down to 7.5 degrees at 5500 RPM and full boost.

With such retarded timing at high RPMs it's no wonder I have no acceleration at speed.

So now for a couple questions -- what kind of timing advance should I be expecting at reasonable engine RPM's? I'd expect far more than I'm seeing, but I'm not sure what is typical since I don't have a baseline that recorded this on my car.

What are likely culprits for my ECU retarding the timing so badly? The knock sensor seems like a primary input the ECU uses to retard timing, so is it susceptible to failing in a way that makes it falsely trigger?

Knut

One more note,
The fact that the lambda sensor read on the rich side doesn't really mean much as they are narrow band so rich is just south of stoich (14.7/1) so even know it is "rich" it may not be rich enough to avoid knock under the load/boost conditions.

 

[mikelr]

There is a main grounding point at one of the upper bolts on the bell housing for the engine wiring harness. Grounding isn't a big problem on the V-8's unless someone didn't put all of the wires back. The only good data logger I know of that can also do snapshots is a Tech 1. Bad power or grounds would manifest itself in other problems too and would not necessarily be so repeatable. One of the subtle clues here is that everything was running fine and all of a sudden this problem appeared and is very repeatable. Something changed and is of such nature that it can't or won't change back. At least you will know when you fixed it! When you have an intermittent you can't be sure for a while, always wondering if it will come back! Was any service work of any kind done just before the symptoms appeared?
David Teitelbaum

Building on what David just pointed out, this is a hard fail with repeatable results. I have to reiterate the importance of verifying your fuel pressure at idle and under load. Ruptured soft wall fuel line in tank is a very common problem with V8's of this vintage. As I stated before the fact that you see a rich condition on your narrow band lambda doesn't mean you have adequate fuel for the higher boost situation. You stated that your IAT's are good so the next best culprit for pulling timing in the fashion you are seeing is knock (which you can not log with your current scanner). I'd verify your fuel supply before going any further, slight fuel starvation seems to fit your symptoms.

 

[mikelr]

A little snip it from the repair manual about the 918's knock sensor...

EMM.2 - T KNOCK SENSOR
The ignition timing required for optimum performance can lead, under certain operating conditions;’ to
detonation of the fuel mixture in the combustion chamber, causing excessive heat and pressures and a characteristic
‘knocking’ noise. If allowed to continue unchecked, major engine damage can occur. A single knock
sensor is mounted in the crankcase ventilation cover at the front of the crankcase ‘V’, and is able to identify
detonation ‘knock’ and produce an AC output voltage which increases with the severity of the knock. The
knock control strategy programmed into the ECM allows the engine to adhere as closely as possible to the
optimum ignition timing and turbo boost settings without a damaging level of detonation.
Knock sensor
When the knock sensor detects the onset of detonation, the ECM first rapidly retards ignition timing to a
safe level, and then progressively advances ignition until detonation is again detected (if at all), and the cycle
repeats. If however, retarding the ignition does not stop the detonation, the ECM also rapidly reduces boost
pressure before slowly allowing it to build up again (see Sub-Section EMM.2 - R).

 

[mikelr]

I don't see how you could really get to it without pulling the upper intake. Besides if you disturb the seals you needs to pull it completely to reseal properly.

 

[mikelr]

What kind of map values are you seeing? I think it would be helpful if you made a list of all the pertinent sensor values during a pull and posted the values here so we could evaluate them.

 

[mikelr]

Refer to my previous post (16)
If any of the inputs mentioned are not within range it would cause the timing to be reduced to reduce power and protect the motor. You are just going to have to examine each sensor to find one causing the problem. Instead of driving the car it may be more useful to use a dyno if you can get on one. My first guess would be IAT and MAP but that should throw codes. Reduced fuel pressure could be causing this and you would not necessarily see an error code.
David Teitelbaum

I get a gold star if that ends up being the problem!