Mrmerlin

its common for the plastic junctions that plug into the MAF boot to crack thus causing an air leak.
They are a lil difficult to replace

Marti

So having repaired the plastic Y pipe at the charcoal canister I had another go at tracing any air leaks.

I have omitted from mentioning in my original post that the car has had Colins Cams installed along with the 30lb injectors.

When I started the engine it was still hunting but did not seem as bad as before. I closed off the pipe from the brake booster with no change. I checked the vacuum test port and the vacuum was pulling some 300 on the gauge. I checked the vacuum reservoir and it seemed to be holding vacuum - the was a long hiss as it emptied - the HVAC is also operational so I think I can discount that.

I disconnected the ISV to see if the car settled to a steady idle with it disconnected but it did not. So I remembered that you can use the Sharktuner to manually change the ISV so I thought I would try that. The ISV did react to the manual changes and when I switched the RPM to the GT setting the car idled much better but still felt rough but hunted much less.

I had not expected the engine to struggle to idle with the bigger cams at the S4 idle speed or display the hunting behaviour so I am not convinced there is not another issue.

I have ordered a complete set of pipes which connect the oil breather / ISV and the MAF boot to ensure I can eliminate these

Any other ideas? -

It has been a long time since I properly the ran the car given the extent of the restoration / modification

could the ECU have gone bad?

Marti

I am beginning to wonder after fitting the new cams whether this could be contributing to the rough idle

FredR

Marti,

The cams may well play a role and Colin should set you straight on that one. Given you are using ST2 you can try what I did which is to set the AFR at 14:1 and add a couple of degrees or so of advance. I also set the cells around the idle point at the same AFR and advance as the operating point seemed to wobble around two cells. You also need to set the CO pot at the default resistance position but I cannot remember what that is [350 ohms?]. Whatever the correct number it enables some mixture adjustment. I understand that 14:1 gives the correct CO number . If using ST2 then the idle pot to some extent is redundant.

I use Roger's EIS inlet system. I found during experiment with that that the idle is somewhat fickle. Putting a honeycomb in front of the MAF helped the idle to be more stable but it also seemed to choke air flow a tad so I dumped that. Given this fickle nature I would not be surprised if Colin's cams add to the issue if reversion pulses play a role in this. One also wonders if your inlet manifold mods might be contributing given the system is so fickle.

My idle bumps a little but the rpm swing's cannot be discerned on the gauge it is so small now. ST2 should give you an accurate range of values should you wish to record such. Bottom line- the perception of a bumpy idle probably varies from owner to owner- things like that irk me but many would not be bothered or so I suspect.

Marti

Does anyone out there know the CO Pot default resistance?

Marti

John Speake

The factory setting is 386 ohms, I set my rebuilds to 300 ohms to give a bit more range..

Marti

John Speake

300 ohms

Speedtoys

"I found during experiment with that that the idle is somewhat fickle. Putting a honeycomb in front of the MAF helped the idle to be more stable but it also seemed to choke air flow a tad so I dumped that."

Yes, Rogers system..no real fault of his, does not product a smooth linear flow of air over the whole of the hotwire, which is problematic at any flow, but more noticeable at idle. But given this is SUPER common amongst a lot of high HP builds, im curious how you saw it as a flow issue.

Theres sufficient flow for 100s more HP than the stock engine needs thru there...even with a honeycomb.

Speedtoys

I had his cams on a former S4, it needed a slightly higher idle for these cams, but there was no "hunting" issue with or without another 50-75rpm of idle.

FredR

If the inlet system has an air leak the rpm's will naturally rise and the mix will lean out proportionally. If the engine speed rises the ISV will try to compensate until such time as it is out of range. If the ISV nips up then measured air flow will reduce. If one disconnects the ISV then if there is a manifold leak, the air path is fixed and the revs will rise. This is why I suggested to Marti to disconnect the ISV- he did and the behaviour did not change- de facto the problem is not a vacuum leak. In any event, why should a vacuum leak cause the engine speed to surge? The ISV will nip up until it can nip no more but I do not see any reason why the things should be opening and closing to cause a surge.

I suggested that Marti check with Colin about idle speed for his cams-why? Reversion pulses upset the MAF readings and why John modified ST2 to run on MAP and thus help stabilise idle rather than resort to higher idle speeds when using lumpier cams.

For the idle to surge the air flow has to be changing and this could be down to the ISV modulating but when the same behaviour occurs with the ISV disconnected then the dynamic has to be caused by something else and the only logical reason I can think of is reversion leading to flow resonances. Now when using ST2 I noticed that at idle the operating was point was floating between two cells on the same load range and different rpm cells. Stick the honeycomb in and the amplitude dropped and the idle was noticeably more stable.

I recently changed out my old ISV for a new one. The idle is now a little more stable than it was but there is still a small fluctuation that I can live with. If I put the stock inlet system on it might be a tad better still. I also run with a GTS inlet cam and the S4 exhaust cam- maybe that plays a small part.

I have seen no evidence to support your observation about metering issues with the EIS inlet system at higher flow rates but that does not mean they do not exist. The beauty of the ST2 system is that one can simply tune AFR to whatever is desired and that takes care of any metering issues.

Speedtoys

I spent a lot of time watching the '+' in the live MAF map with EIS and one of the RL S/C kits with a 'direct dump' into he MAF, and it wanders around at steady speed/cruise and idle.

With a honeycomb this wandering around the cell/adjoining cells...depending where you were in the chart...went away. The hotwire just isn't seeing an even consistent flow of air over the whole wire at any given time..to provide stable feedback to the ECU.

FredR

I did not see a need to try and understand what was happening at anything other than idle-one only has to look at the AFR value spread in a given cell. The acid test was running with the ISV disconnected so that it defaults to a fixed aperture. The idle was still bobbing around so flow had to be modulating for some reason. The thing that baffled me was that initially I might have expected flow to be more uniform with the EIS configuration compared to that torture chamber flowpath of the stock airbox plenum. In the end I figured the cause was localised change of velocity. The stock air filter probably acts as a flow damper of sorts whereas the EIS has two roughly equal flow streams joining nicely but as they merge the flow speed doubles right in front of the MAF and has not had chance to settle by the time it hits the hot wire- the honeycomb helps resolve that thus possibly why the idle was a bit more stable with it. The basic rule of flow metering is to have a nice straight flow run for 15 pipe diameters upstream and 5 diameters downstream- no chance on our motors. Interestingly on their big vee 8 motors MB used two MAFs as I recall doubtless in a better configuration.

Bottom line I reckon both systems have a degree of flow instability due to system geometry and that may explain why the 928 idle is just not as smooth as it could be.

Marti