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760 B280F lacks power, prematurely shifts down

tomasss

tomasss

former PRVert
Joined
Oct 13, 2008
Location
Sydney, Australia
Hi guys,
I have a problem with my 760GLE, aw71, 88, equipped with lpg - the automatic trans is shifting down too early, (because) engine lacks power.
List of things I have changed:

Ignition:
new sparks,
spark wires,
distributor cap,
distributor rotor,
both knock sensors,
checked coil - both primary and secondary impedance ok,
cleaned connection between body and power stage,
tested with another ignition ecu,
all connectors cleaned.

Fuel system:
new bosch o2 sensor,
new fuel pressure regulator,
new coolant temperature sensor,
new thermostat,
tested with another fuel ecu,
tested with another air mass sensor.

Intake:
Throttle body cleaned,
idle valve cleaned, also tested with another one,
cleaned crank case ventilation,
TPS sensor work fine,
throttle and kickdown cable are adjusted correctly,
checked for vacuum leaks.

And of course all filters, oil, ....
Valves has been adjusted.

I have installed led diode to the O2 test port - and it toggles (so mixture is correct, not too lean or rich).
Idle is smooth, no disturbances or surging.
But the diagnostic diode I have installed toggles twice when rpm/s are over 2000, and this is code for engine knocking. But engine is NOT knocking. At 2500 and higher, it doesn't toggles anymore, but as soon as I return to 2000+-300, it starts again.

My opinion is that the transmission shift down too early because it "feels" engine has no more power at the higher gear, but I don't now too much about transmissions.
This prematurely shifting is both on lpg and gasoline.

My last chance is, that the fuel injectors are leaking (when running on lpg, the injectors are disconnected, but the fuel pumps are still running - to keep the system pressurized). But can it create a sound like engine knocking? :omg:
 
If you lock into 1st gear, does it still lack power? Is there any point when it transitions from having very little power, to suddenly 'full' power, while your foot is on the gas?
 
On 1st gear, it seems to me as it doesn't lacks power. Also on full throttle (kickdown), it seems to me as accelerating well, but I am not 100% sure with that as I am living with this issue too long. Between these two situation, engine power is pretty bad.
 
This sounds very similar to when I had two ignition issues: Spark plugs gapped too wide, and a spark plug wire loose. Car would miss at idle (not a symptom in your case), and miss all the way up to about 1800 rpm, when it seemed to "wake up" and give full power.

If you put the car in park and rev to about 1500, will it hold at that speed, or does it fluctuate? You mentioned you tested the coil- do you have a spare that you know works well in another car? I don't have any experience with the V6's, so I don't know if it uses just a standard Bosch coil or not.
 
The sparks plugs are new, and it was also bad with the old ones...
Spark wires are new, the same was with the old ones...
There are no problems with idle speed even if it is 800 or 1500...
The coil is standard Bosch, it can be defective although both impedances are ok? Some kind of leakage when operating? I can use coil from friend's car for test.
 
Sorry, you can disregard my idea about the coil.

Another thread (with a vaguely similar issue) I replied to made me think of one more thing. Though you said you checked for vacuum leaks, that doesn't rule out vacuum blockages. Is there a vacuum line running to your distributor or ignition controller? If so, has it been replaced in recent memory?
 
No, to the distributor leads just the spark wires and coil wire.
Ignition controller - you mean the ecu? There is just one big electrical connector connected to it, no hoses.
 
What about the knocking? When it is detected, the ignition timing is retarded so the engine has no power. But what can cause the knocking, I rerouted the wires from the alternator and also installed ferrite cores to the knock sensor wires, but it does not help - so it is probably not caused by electrical interferences, but by mechanical...?
 
Piston slap can look like knock.

I think you should kill the fuel pumps when you are running on LPG. This will extend the life of the pumps. It doesn't take long at all to prime the system once the relay is activated.
 
Switching off the pumps is scheduled work, I am just waiting to better weather...
But can the leaking injector(s) cause some signal like knocking?
Ad the piston slap - if it is the cause - what is the piston slapping into?
 
Hmm, when knock sensors are disconnected, ignition ecu knows it is disconnected or faulty and retards the timing all the time...so, no, it doesn't improve the issue.
 
Last edited:
tomasss said:
Hmm, when knock sensors are disconnected, ignition ecu knows it is disconnected or faulty and retards the timing all the time...so, no, it doesn't improve the issue.
Click to expand...
On your own risk, of course, you could try bolting the knock-sensors off, leave the connnectors attached but just unbolt them and tape to some hose or something?
 
TVS said:
On your own risk, of course, you could try bolting the knock-sensors off, leave the connnectors attached but just unbolt them and tape to some hose or something?
Click to expand...
No, this is not the way. The ecu expects some kind of signal coming from the sensors, when I unbolt them from engine block, ecu is signalizing faulty sensors.
 
tomasss said:
No, this is not the way. The ecu expects some kind of signal coming from the sensors, when I unbolt them from engine block, ecu is signalizing faulty sensors.
Click to expand...
OK, that just worked with a bit older model EZK, as the sensor that was attached to the block died, I just put another one hanging in there so I got the missing 30hp back till I got back home to replace the original one.
 
I think they are suggesting this only to see if the computer is pulling timing. If it is then you will know it is a detonation problem.
 
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