We bought a discovery and found out that it needed a new head gasket. As I was driving while the engine was still warming up when I would accelerate right around 2,500 rpms I lost a significant amount of engine power check engine light would flash and then would gain power and run fine. So we decided to get it done.
We took this to a Good Year shop that said they have worked on many of these. $1850 later with a new head gasket, plugs and machined heads we left the shop. It still was exhibiting problems, so we purchase an ODB2 bluetooth scanner for $20.00 and used the Torque Android software to read codes. Coming up as O2 Sensor issue and still lack of power when accelerating with cylinder misfires. so we took it back to the shop and they replaced both banks in the front. We took the vehicle home and was still running into issue when cold, so we took this back to the shop, they then believe it was the Mass Air Flow sensor, but had no way to simulate / test this so they got a remanufactured unit they could put on to see if that was the issue. They said that it seemed to run better but didn't fix the problem. Still lack of power around 2,500 rpms. Good Year then suggested we take this to Bavarian AutoHause (specialized in european cars) as they would have better tools to diagnose. Bavarian said they could diagnose for $770. During the teardown they found Plugs not connected spotty wiring job, over tightened Air Intake Manifold (should have been in the 14 ft-lbs of torque range, this then warped the gasket and caused damage to the air intake manifold it self. The Good Year shop agreed to pay for a new remanufactured AirIntake manifold and gasket, new Bosch plugs and wiring (those should have been used, vs the cheep ones used before). Bavarian fixed this up drove it around and said it was ready to go. We picked it up drove it home and on the drive home the Engine light came on again while accelerating Plugged in the ODBII scanner and it was coming up as cylinder misfires. The shop then spent the next 2 weeks trying to ID what the issue was (they tested compression--no problem there, they tried Mass-Air flow sensor--no issue there, they tested Coil packs--no issue there), the only thing he could find out that was wrong with it was Vacuum loss at Idol. Specifications say it is suppose to be at 20 lbs or s and is coming in at 14 lbs (I think he used lbs as the measurement cant remember exactly (according to Bavarian AutoHaus) and that 6lbs loss is significant. He said without tearing the down the motor completely there was no way to find out what is causing this, and that this would be thousands. The fix he said was a new/reman engine with his labor installed $15,850!!
Any Ideas The car still runs fine once it heats up just still have a power surge around 2,500 rpms.

 

I am surprised none of these experienced technicians checked for a restricted catalytic convertor. Remove the upstream 02 sensors with engine cold and take for quick run up to 2500 rpm. If it accelerates properly they are restricted.You could also attach a compression tester in the 02 hole (same threads) and run the engine at idle. should not have more than 5-8 lbs pressure. I have had some read up to 35 lbs!!! Restricted convertors can also cause vacuum loss.