CIS running lean at high RPM
 

Looking for some expert advice. I have been working through this problem since September. I have two shops scratching their heads. Over 70 years of 911 experience. Top notch race shop.

The subject:
1978 SC track only car with Euro engine
Rebuild March '09 (has run perfect until September '11 - about 15 DE weekends)
Punched out to 3.2L with 9.5:1 compression
964 profile cams
pre-'74 headers
Factory CIS

The problem:
Engine idles and revs fine when static
Under load (track or rolling dyno) the engine runs very lean (40+hp loss) above 5,000 rpm.
When cool the engine will rev over 6,500, but with each subsequent pull it falls to finally around 5,000 RPM
Dyno results show air fuel mixture going dangerously lean

Tests and work performed
Leakdown - 2%-3% on each cylinder
Compresson - as expected on a healthy engine
Ignition timing at high rpm - consistent and where it should be
System fuel pressure in all states is within tolerance
Fuel injectors cleaned
New injector sleeve and 0-rings
Covers removed and valve springs eyeballed - all good
Complete new ignition (MSD) (done early before lean AFM was known)
New Magna-core wires
New fuel pump - filters - blow out the lines.

Yesterday a complete different CIS was installed (a know good unit). From the fuel "In" line to the manifolds. Same result on the dyno. In addition, the entire car's fuel system was elliminated by using an indepedant fuel supply, pump and lines.

So. This leaves the long block. Its been checked for bad studs, valves and springs, leak down, compression.

We are procuring a set of Webers to put on the engine and see what happens. Won't get this done until late next week

Speculation as to a broken ring causing a loss of vacuum or some other condition that may effect CIS operation at high RPM. There are no tell-tale symptoms of a broken ring. No smoke, puffs, miss-fire, ect., however.

This is getting expensive. Hoping to hear from the experts on this forum that may have experienced this very problem or possible next steps short of pulling the motor a part.

Apologies in advance for any mispelling. Could not get the spell check tool to work.

Plugs, wires, coil ?

That you are going lean at high rpm means that the air metering plate is lifting with air flow but fuel is not being dispensed proportionately? Either that or you have a massive air leak which you have ruled out by completely changing out the CIS assembly, and presumably fastening everything together as it should be. Why would you be getting plenty of air but not enough fuel? Since you have cleaned all the fuel lines, replaced the entire CIS system including the fuel pump and filters and the car was working fine until September, how about a blockage inside the fuel tank? You are experiencing fuel starvation and have checked everything else. If this were a carbureted car I would say plugged fuel filter, giving loss of power under load. The only other place there could be a fuel blockage is at the tank itself. Maybe a piece of plastic or cellophane got into the tank and is preventing fuel flow to the fuel pump? That used to be a popular prank, small pieces of cellophane in the fuel tank. Almost impossible to find.

Cam timing

I would think a fuel flow rate test would eliminate any blockage or fuel pump worries and that surely has been done by now.

Check the cold enrichment valve for leaks and the cold enrichment vlave thermal circit operation.

I don't know about high performance engines but it would seem to me that the injection system would need some modification with different cams, larger displacement and higher compression. Are 964 cams installed on CIS engines? You'd think the modified engine would need more fuel delivered at high RPMs than the original CIS could provide. I think even-though you're drawing in more air to raise the plate it still might no be enough. Have you tried different injectors that have more flow? Again, I know nothing about racing or high performance engines just trying to give you some ideas.

All,

Everything but the long block has been eliminated using a complete different CIS, remote fuel cell, pump, accumulator and lines. The result was the same. Very lean AFM above 5,000 RPM. "System" fuel pressure is within range at all of the variable conditions.

Will CAM timing effect CIS fuel flow (withing the limits of non-interference)? This seems logical since the pulling of air through the intake could be disrupted by poor valve timing. I am told that the ignition (with timing light) is stable throughout each pull. The dizzy is driven by the left side cam. Yes? Perhaps the right side cam is going out of "time" due to chain slack at higher rpm? Perhaps a chain tensioner is worn? How best to test the theory?

This engine idles perfectly and revs without hesitation when static (cold or warm). Additionally, the problem appears to effected by engine temperature (under load). When cold (adequate warm up) the first dyno pull is not bad (6,500 RPM). Each pull after that gets worse until it settles in at around 5,200 RPM before falling off (its a drop of over 45HP). When driving, it feels like the fuel got shut off much more than an ignition miss. Back off the throttle slightly, and the engine picks back up or maintains its RPM around 5,000.

Keep those thoughts coming. I suppose enough time and money will eventually fix it even if that fix is 3.6L variocam engine swap.

get an adjustable wur. the stock 55lb spec is your prob. with your engine mods. I basically have same engine, and was lean on the dyno on the top end. I dial down to around 45 lb when driving hard. remember more press =lean....less press = rich. I just use an aero manuall press reg mounted between my seats with a big knob.,, and dumped that prehistoric stock wur. AFR adjustments on the fly with K=jet works great.

Of course you will have to have a gage to monitor afr, .http://forums.pelicanparts.com/uploa...1327791510.jpg

http://forums.pelicanparts.com/uploa...1327792464.jpg
top gage is AFR . lower is control press.

PM sent

As Don wrote, you need to measure your control pressure,.......that's the key to high-RPM, WOT enrichment.

This car/engine has run quite well with the set-up described for three years and 35 track weekends. I believe the WUR System pressure to be currently in the mid-forties. The other CIS that was installed used its own WUR with the same result. Thanks for the information and I will pass your tip along and see if this adjustable WUR is a potential solution. I am also hoping to borrow a PMO set-up soon and see what happens on the dyno.

an adjustble control press reg. is the only solution to get the max out of cis. other than that, you are stuck at what ever you warm press is and a fixed afr. Sure make a differnce between hot and cold days, and on the dyno. On the freeway, i dial up to 60lbs, gives me 15-15.4 afr, which in turn give me 28 mpg doing 70. About the same as my new 4 banger 5 speed toyota tacoma

Are you saying your AFR issues are a new problem and the engine used to perform correctly?
If so you don't need more fuel but rather to just find out what changed, yes?
If not then your fuel head can be adjusted/modified for more flow as well as the WUR made adustable. We've done a few 3.0L race cars and they do need a bit more fuel than stock.

Correct Rarlyl8. A new problem Cannot get the engine to run more than 5,200 RPM with two completely different CIS. Mixture has been richened considerably at idle and result is the same. The engine with CIIS, 964 grind cam and headers has been on the track since 2002. Its been a 3.2L since early 2009. Never a problem until late last year and we have been chasing it since.

turborat - what was the result of this? Did you get it fixed in the end?

Chasing a similar issue with my car (I think). 1980 SC (US) that I've fitted SSI's and a Monty 2in/1out to. No problems prior to the exhaust being fitted, now loss of power over 5K.

PM sent.

curious too, did you fix it