91 TR acceleration and tachometer problems

Continue on the restoration of my 91 TR, more problems was found. Hope some experts will shed some light on it. Thank you in advance! The car runs with the original pre-cats and cats, but the o2 sensors and exhaust temperature sensors was unplugged by the previous owner for some unknown reasons.

When the engine is running around 2500 - 3000rpm, the car jerks, feels like the fuel is being cut off and back on within half a second for a few times. It is more serious with heavy throttle. When applying light throttle, just slight hesitation on acceleration. When the engine revs above 3000rpm, it's back to normal.

When it reaches 4000rpm another problem occurs, the tachometer stops there, even though the car keeps accelerating. A local workshop tried to hook up a spare tachometer for testing, the spare gauge performed the same.

Any advice on how can I diagnostic these problems. Thank you.

 

Thank you for the reply.

Yes, RPM gauge, it stays at 4000rpm most the time, but have seen it dropped back to 3000rpm couple of times.

 

With help of a mechanic friend, tried a few things on the Testarossa. Reconnected the O2 sensors, engine vibrated a lot more, the exhaust smelled very rich, 2500-3000rpm still jerked and above 3000rpm no jerk but no power, same as the O2 sensors unplugged. Checked the O2 sensor voltage, the one of the senors might not work well, ordered a replacement. However, I think that is a separated problem not related to the jerking. Also checked and adjusted the air fuel mixture, checked cables, replaced the spark plugs and fuel filters, the engine feels a bit more lively, but the jerking and lack of power problem still exist.

 

Checked the cables, connections are good and all plugs are firing.
No, I'm not aware these the special cables. So their condition can't be determine just by spark plug firing? Need changing regardless?

 

Thank you all for your advice. We are focusing on the acceleration problem, haven't touch the RPM gauge yet. Will try to inspect the fuel system first before working on ignition after the basic checking of both.. So far, found that the problem was with bank 7-12. It only happens under load, cursing at 2500RPM then heavy throttle will trigger the symptom, gentle with the accelerator the jerking will not present. If running with all 12 cylinders, it will not happen in 1st gear, only in 2nd gear and up. It seems to get worse when the engine heated up. After 3000ish RPM the jerking goes away, but lack of power. We have replaced the fuel filters (one was bad), replaced all the injectors, installed new O2 sensors. The engine feels most strong now. However, the jerking still there. We try to measure the fuel pressure of the system. the primary pressure is 5.2bar, the pressure before entering the injector is 3.2bar at 1000rpm, Fluctuating randomly between 2.2bar to 4.0bar at 3000rpm. Is that normal? Since we don't have the additional D section in our manual... Are those two fuel distributors the same, could we just swap them for testing?

Thank you!

 

Thanks Turbo-joe, what's the pressure before entering injector should be at 3000RPM? Should it remain constant?