Some time ago I posted a runability issue with a customers Esprit turbo. Now We are Ferrari guys and have a limited expierence with turbocharged cars.
Our issue was on hard accell around 4000 rpm, the engine seemed to be rev limiting, like the old rev limiting rotors on early German cars. The power curve seemed to hunt, power up, and then shut down, power up and shut down.
Being Ferrari guys we know a thing or 2 about cam timing and timing belts, so we degreed the cams and installed a new factory belt. We replaced the cap, rotor, the pick up in the distributor, new plugs, wires, coil, and ignition module. None of the above made a bit of difference. Next we checked and rechecked everything in the fuel injection system, pressure, delivery, relays water in the fuel system, all checked out perfectly.
We had a few responses to our previous post here, we followed all input and it did not make a slightest bit of improvment. The interesting thing was all emissions were spot on, all voltages exact,
Our next thought was an issue with the turbo. Turbo was in perfect operating condition and free running. We checked the hard line between the turbo and the pop off valve, no carbon.
Then we checked the pop off valve and it seemed a bit sluggish in its operation. I ordered a kit, and a new valve and spring. We installed the above parts and still the same problem extisted. We even removed the exhaust system to make sure we did not have a plugged exhaust, no change.
We reposted and no new information offered. So I decided to just drive the car on the freeway and watch the gauges to see if there was something that we may have missed. If I throttled up easily beyond the afore mentioned 4000 rpm, pressing lightly on the throttle it accele
rated fine. If I pushed the pedal hard it ran up to 4000 grand then it started to rev limit on and off, on and off.
Thats when I noticed when hammering the throttle the boost jumped up + 11 pounds and the rev limiting started. I installed a test boost gauge to double check the one in the car. The numbers and results were the same.
So my conclusion was for some reason the boost was exceeding the built in limiter and it was shutting the fuel supply off, not long enough to starve it but just enough to cause a hesitation under hard accell.
We went back to the turbo and rechecked everything, and finally found the problem. While the pop off valve was operating perfectly and worked as intended. What we did find was a worn valve stem hole in the cast iron pop off valve housing. It was worn oval and was causing the valve to stay shut under load allowing the turbo pressure to build up and shut down the fuel supply.
We removed the valve and machined the oval stem hole round, then we machined a phosphor bronze valve guide and pressed it in place. We then reemed to fit, with the heat generated in that area we allowed more clearance than we would with engine valve guides. Due to heat expansion of both the valve stem and pop off valve housing we allowed .007 clearance.
We reassembled the valve and fired the motor up. Took it our for a quick road test and problem solved. The car is now a rocket, fast hard throttle accelleration up to 7 grand. Our initial problem seemed to get worse with higher temps so we blasted up the freeway for 30 plus miles and then returned. This Lotus runs strong and smooth with very nice powerful acceleration, at running temps.
Its now a nice fun car to drive with steady accell and it does run strong. Sorry to be wordy but with these cars getting a bit long in the tooth I'm sure others may or are expierencing the same issue. Turns out to be a rather inexpensive and easy fix. Once you remove the pop odd valve any machine shop can install bronze guide and problem solved. By the time you throw in a new valve, a pop off repair kit and new spring, the whole project should come in under $500.00.