Another Oz-nology post for you guys. This one is regarding IMRC's, and more specifically IMRC delete/lockout. This was a reply to an email from a customer that was experiencing rough engine running for the first few seconds on a cold start on a 15-17 F150. The truck is basically stock, but had an IMRC failure, so they were locked out and the truck was tuned by us for the IMRC lockout. We had the customer record some logs to make sure there wasn't a bigger issue like a vacuum leak or something similar, and everything was fine mechanically. So, I shared some knowledge with the customer after making some adjustments for him.
V3 is in the cloud. I made what changes I could, but this behavior is part of the PCM’s emissions control logic. Normally, when a truck has IMRC’s, the IMRC’s are closed immediately after the engine fires while the VCT system is in its emissions control phase. This is done to make the intake port cross section smaller, which speeds up the air entering the engine. Faster moving air helps to improve engine vacuum, increase charge-fill into the cylinder, and also reduces fuel puddling on the wall of the intake port.
The fuel puddling is the bigger issue, especially when running ethanol. Reason for this is that gasoline and ethanol have drastically different flash points. Flash point refers to the minimum temperature at which a liquid gives off vapor in sufficient concentration to form an ignitable mixture. Ethanol’s flash point is 55*F, meaning it has to be at least 55*F to be in vapor form, whereas gasoline has a flashpoint of -45*F. That’s a massive difference and remember that fuel does not burn in liquid form, it will only burn when in vapor form. Also, that 55*F flash point doesn’t refer to ambient air temperature, it refers to the temperature of the liquid itself. So, things like spraying the fuel through the nozzle of the injector where the fuel is compressed through an orifice and then expands will cause the fuel to drop in temperature because expansion naturally causes a reduction in temperature. So, even if it’s 65*F ambient temperature you can still have issues with vaporizing ethanol because of the temperature changes that occur throughout the injection process. Fortunately, engines tend to heat up pretty quickly after they start running. So, it doesn’t take long for the walls of the intake port to become hot enough to vaporize any liquid fuel that might be hanging out on the port walls, but until it reaches that point the engine is not going to be very happy trying to burn liquid fuel. Since you no longer have functioning IMRC’s, the easiest solution is just to run less ethanol when ambient temps are cooler. You won’t lose any performance by running less ethanol, even if it were 110*F ambient temperature. As long as you've got around 30% ethanol content, the engine will produce maximum power. If your curious to know why, I made a post a few weeks back explaining how octane is relative to performance here - [https://www.facebook.com/groups/hp5.0f150s/permalink/2599282563614265]
The IMRC’s are Ford’s solution to this. Another solution would be to run a camshaft with less lift and/or duration or with a later intake valve opening event. The issue with any of those options is that they would all have the negative impact of reducing the engine’s total power output, especially at higher RPM’s. This is why Ford decided to use IMRC’s. It’s a simple, inexpensive, albeit low-tech way to make a “big” cam run like a smaller cam by increasing the velocity of the air charge through the intake ports in the load and rpm ranges where the cons of a larger cam outweigh its benefits. Other manufacturers have other ways of achieving the same thing, like Honda’s VTEC, which actually uses to separate cam lobes. One that is low-lift and the other that is much higher lift. VTEC does work, but there’s no way to smoothly transition from one cam lobe to the other, which is where the term “VTEC kicked in” came from and why those earlier Honda engines would have a very abrupt change in engine sound as well as acceleration rate. Personally, I think Ford’s solution is a little more elegant, albeit not as boy-racer sounding, but I’m digressing. This is why we don't recommend deleting IMRC's if there is an option to keep them active as there is no performance benefit in doing so. Obviously with no Gen 2 F150 intake manifolds available from Ford, that isn't an option. Hopefully, now that I've given you some information about how they work and what their purpose is, it will help you have a better idea of how having them locked now can impact how the engine runs.
Ken "Oz" Osborne