Hello Lucas, Before you consider spending loadsamoney upgrading for performance, your first objective should be get the engine running at its best possible efficiency.
I endorse what kiwicar says about checking the key sensors such as the coolant temp sensor, throt pot settings, fuel pressure regulator, etc, but you probably (potentially) have the thick end of 150-200 horses under the bonnet so you deffo dont need to pay out good money to add any more, before harnessing what you already have, for a successful towing performance.
13 mpg with black plugs on a fourteen year old engine, that may have even more problems (than the predictable throttle pot replacement) is clearly wrong.
I also detect that you have not had the car very long (5 minutes?), but really want to improve it and keep it.
You said you want to give it a proper service, which sorta suggests it may be a tad overdue! If thats the case the proper service needs to include a number of action NOT normally undertaken by service agents or prior owners/home enthusiasts.
Things such as breathers, plenum cleanliness/throttle adjustment, ignition cleansing, plenum air leaks, etc, may never have been attended to by previous owners and, be sure of one thing, these things are neither weird nor unsusual, especially when dealing with the RV8 Efi's, which are much less tolerant of poor maintenance than the carb versions.
I suggest my 15 point program to regain all the power that has potentially been lost due to age related deterioration, possible lack of preventative maintenance and even prior neglect. Brutal suggestions, I know, but not unusual!
Whilst not written for your actual RV8 Efi engine, The principles are sound and all the recommendations could be easily adapted to your particular Ignition and Efi systems.
Rover SD1 V8 Efi Power Loss Recovery and Health Check Program.
Assuming the engine is in good mechanical condition with a working ignition system, correct cam/lifter performance and cam timing, good compression within acceptable limits on all eight cylinders, good exhaust system with no leaks, fresh lubrication plus clean filters, then follow these fifteen steps to ensure the ignition and fuelling systems are in good health.
1 Clean all ignition components externally and internally, spray with moisture inhibitor such as WD40 and wipe dry. Ensure the distributor vacuum and mechanical advance mechanisms are lightly lubricated, working correctly and the springs/bob weights are free of any corrosion.
2 Check security and eliminate any corrosion on all ignition Low Tension connections right back to the ignition switch, including all local earth connections.
3 Clean and re-gap the spark plugs - replace if necessary.
4 Check for damaged or out of specification (measured suppression resistance) High Tension leads, Pay particular attention to the rubber boots. Clean and replace if necessary, finally checking for nice fat sparks at the plugs.
5 Set ignition timing to just eliminate pinking with normally available fuel. See here for a neat process.
(Note: Items 1-5 are to guarantee and maximise the best possible spark at exactly the right time. Check workshop manual for recommended component and timing specifications.)
6 Ensure the plenum idle air gallery is clear and not contaminated with goo.
7 Thoroughly clean the plenum breather gallery, flame trap and LH rocker cover breather filter
8 Ensure the throttle disc(s) is correctly seated at idle (or set to the recommended disc to tunnel gap where specified) to prevent "idle speed hang-up" and that the whole throttle mechanism is free of unwanted friction right through from accelerator pedal to throttle pot.
(Note - items 6-8 can be best performed by taking the Plenum Chamber to the workbench). Go here for Plenum Solutions.
9 Set the AFM CO content to the recommended range erring just on the weak side of the mid-point. If a CO meter is not immediately available set the adjuster screw to 2.5 turns out, from fully home. See here for AFM stuff. Reset idle speed to the recommended level. The settings can be interactive, so recheck both.
10 Set the Throttle Position Sensor residual voltage to 325mv (some spec's call for 350mv). Ensure the sensor is not electrically noisy, causing random overfuelling. See here for full detail.
11 Verify the fuel pressure operates in the range 26 to 36 psi (for a 3.5l engine) not more, not less. Details here:
12 Test the integrity of the plenum and all its air/vacuum hoses for rogue air-leaks. How To here:
13 Check all Efi loom wiring/connections for faults/corrosion including the Efi engine earth AND the engine earth strap. Unusual Efi Earthing Issues here:
14 Check and replace air and fuel filters if necessary. Ensure fuel tank is not generating crud.
15 Because the Plenum would be removed for proper cleaning and accurate throttle plate adjustment, lift and service the injectors, replacing filter baskets, pintle caps, fuel pipes (optional if damaged) and injector to manifold seals. See here:
The program is designed for owners, generally unfamiliar with the ignition/Efi systems and for cars that have been subject to longer term neglect and/or under-use.
Selecting which points to omit is perhaps OK for owners thoroughly familiar with their engine systems, because longer term ownership may lead them to believe certain issues have been previously dealt with. Generally though, when something just "seems to be OK", it equally might not be!
