P0172 & P0175

Hi....I'm from the UK and have a Lexus GS430 MkII, 4.3 V8 auto, 108K

2 months ago, driving along in heavy rain and Engine mgmt light comes on.....car appears to run rough.
Codes of P0172 & P0175 reported by recovery company. Recovered to Lexus dealer.

Call from them asking if I've been driving through deep water...as they said they found a lot of water around the engine bay and some in one of the cylinders. They also stated possibly coolant.
Also reported I had low compression on that cylinder (90) They then said they suspect I had a bent piston due to the water - wanted to charge me 1K GBP to look - 2K if it was bent and needed fixing.

Declined the offer and took elsewhere - When I collected form Lexus noticed the light was no longer on - not sure if they cleared or gone out itself.They tested compression - and all was fine (around the 150 mark on all cylinders) and replaced the sparks.

All well for a few weeks - then happened again. Also noticed a "shaking" as if someone was thumping the car every couple of seconds....a bit like a missfire but dont think it is. Its never immediate - always starts about 4-5 seconds after I have stopped at a junction. Doesnt matter if I leave in drive, neutral or park. RPM wavers a little and couple of times has dropped to about 200, nearly stalling - but never actually has.

On both occasions I had just parked my car on a large incline and it had been raining. Suggestion from someone was that water was somehow getting past the coils, into the cylinder. I had read about this on other cars and thought possibility.

Replaced plugs again - few weeks later, had been dry and hot all week (unusually). Had to park on slope at office - got in car to go home...light on again. So puts the water getting past the coil theory out.

Local garage now sugegsted replacing the precat O2 sensors as they may just be faulty and engine not running rich at all......but not entirely sure myself because of this "thumping" at idle, would both O2 sensors fail at exactly the same time and they went few weeks ok - then fail, few weeks then fail again - both at exactly the same time - unlikely?

Think it could be something to do with the Fuel Pressure Regulator? or Fuel return line (blocked/restricted)?
I'm wondering if the so called "water/coolant" in my cylinder was actually fuel?

Any advice appreciated....costing me a fortune in plugs (£18 / each and 8 at a time) and labour....and ruining my dream of owning a Lexus.....

Don't replace the o2 sensors, because you're right, they'd rarely go bad at the same time. Codes don't lie. the codes say that your engine IS running rich, regardless of what the local garage says. Those codes mean that the engine can't take enough fuel away to give the proper fuel/air ratio. Head gasket leaks wouldn't come and go. Check that all your splash shields are in place in the front and wheel wells. Keep that water out of there.
Start with watching fuel trims and see what they're doing. Most likely a leaking injector or FP reg. or something similar.

I am not real familiar with Lexus systems, but I am sure there is some type of air flow sensor. The fact that this is occuring during a rain tends to make me think that your air flow sensor is being affected by the moisture in the air. You could try misting water near the air intake to see if this duplicates the problem.

I would guess it's morelikely that all the water he reported in his engine bay and in his spark plug recesses would be the cause of the randommisfire he was experiencing while driving in a rain. LIke I said, fix the splash shields and then tackle the fuel delivery situation. THere's likely two problems. As for a MAF and humidity causing a misfire? I don't guess it's impossible, but that would be a first for me, as Ive driven through rainstorms plenty of times and never experienced anything like that nor have any of my customers.