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When you cold start a vehicle the ECT sensor works in conjunction with the IACV to set initial idle because the engine is in open loop. I notice you have a code for VTEC malfunction: P1519 - this could also cause the engine to idle erratically.
I would be checking the wiring surrounding the IACV since you've already replaced it, and the passages/ports in the intake manifold. Clean them out good with brake clean.
Have you removed the VTEC solenoid and inspected the filter/screen in the gasket?
I am telling you right now that having a cat-back with a stock cat will NOT cause your problem, not sure how that's even a suggestion.
So, for the benefit of the forum and to possibly save someone else the trouble down the line I would like to provide an update on this issue.
Suffice to say, I have really learned something with this one.
After all the work above and focusing on fuelling, ignition, sensors and other electrical checks I can report back the problem was; oil pump failure!
I have been working on Hondas and especially S2000s for many many years now and have never seen a failed oil pump, until now! It was absolutely the last thing I would have checked. Anyhow, a brief summary.
The car continued all the behaviours listed at the start of the thread, until one afternoon when leaving work the car started, then immediately died. Upon restarting there was clearly no oil pressure. The ‘idiot light’ (oil pressure light) was on and the timing chain tensioner could be heard. After a couple of seconds the engine was turned off and recovered to my garage for further investigation.
The first thing I noticed when checking was that the oil filter was ballooned, very very swollen compared to a new one. I removed this, and carefully cut it open to check for swarf and debris. There was none to be found.
I tried some short cranking of the engine with no ignition, and confirmed that there was in fact no oil pressure at the filter, at all. I removed the timing chain tensioner and VTEC solenoid to check for swarf, none found.
Next I drained the oil through a fine gauze, again checking for swarf. There was a very very minimal amount, and not more than I’ve seen in routine oil changes on other healthy cars.
Then I removed the sump/oil pan. Again, no real swarf to speak of, and certainly no shiny metal material indicating bearing wear etc. Somewhat relieved, perhaps the engine isn’t toast!
So why no oil pressure?
Initially, I could see the oil pump chain was intact, as was the drive gear. The pickup was completely clear of debris. I removed the windage tray and then removed the oil pressure relief piston/spring from the oil pump.
The first clue appeared. The piston did not feel smooth in the bore when moved. If this was stuck open, then we could have low oil pressure, and the oil would be sent back to the sump or if it was stuck closed we could have high pressure, perhaps ballooning an oil filter.....
No alternative for it. The oil pump would have to removed for inspection. This turned out to be the right move!
As you can see, the entire oil pump rotor has shattered into pieces! There is severe scoring to the drive surfaces of the rotor housing etc. How this didn’t jam the rotating assembly and wreck the chain/motor I do not know! Nevertheless, I think we may have found our reason for no oil pressure!
New pump fitted and rebuilt, and the car runs like an absolute dream. Smooth on startup, pulls perfectly and all seems well, fingers crossed!
My conclusion; I think the temperature thing (up to 20°C) was the key point for the oil viscosity change. When the oil was cold in our U.K. winter, and ‘thicker’ the stuck oil pump relief and pump itself were pushing the oil at very high pressures, swelling the filter etc. When this was completely swollen, the pump could not force the oil any more so the strain was then taken up by the idling engine, causing it to surge as ‘lumps’ of oil are pushed at pressure. When the throttle was blipped to clear the surging, the additional torque possibly overcame the stuck piston and things began to work as normal, clearing the surging.
Alternatively, if you didn’t clear the surging then the oil would warm, and then thin as the viscosity changes, this made it easier to push round the system, and possibly thin enough to lubricate and free the stuck relief piston. Thereafter, surging stops.
This potentially carried on until the oil pump vanes could take no more and sheared due to metal fatigue. I assume once one lobe was lost, it would then have killed the rest of them. I also assume this was the exact moment the engine stalled on the most recent journey before we had no oil pressure.
So do S2000 oil pumps fail? I guess they do, based on what I know now. Although I still think it’s uncommon. The blessing in this case was that it let go within seconds of startup, doing 1000rpm or so. Had it failed at 9000rpm I suspect it wouldn’t just be an oil pump that needed replacing!
I wonder how many dead motors had failed oil pumps whilst on track etc but since the damage thereafter was so sudden and catastrophic the oil pump never got looked at properly, or was written off as a casualty, not the cause.
In hindsight, I wish I had connected an oil pressure gauge to the engine when it was surging originally, that might have led me to the problem quicker. But as I said earlier, I have never begun investigating a cold start idle problem by looking at oil pressure!
Good learning for me I guess! I hope this is useful for anyone else that encounters this problem, and good luck!
02-10-2021, 04:27 AM
