There are many things you can do - it's like peeling an onion on older cars. Some things need to be done but won't show an improvement if other things also need to be done.The car runs great but once I stop at a sign the idling is rough when the engine is still cold. I don't have any lights, but I notice my mpg is down to 43.3 which is very low. I used soform for clearing the valves which now I'm at a lost on what the issue might be. Could this be a timing or coil problem? A vacuum leak somewhere on the engine. If anyone would like me to check certain items please provide me with the location and a picture if available because I'm very new at locating components on insight, and any suggestion on what can be causing my mileage to get so low.
First, edit your profile or signature so that we know where you are - maybe you can meet up with someone local!
Next, I just went through the "pour stuff into the tank" exercise including several thousand miles on premium for the additives. It's easier than tearing down the engine but it will only get you so far.
For example, those pictures you see at the gas pump of valves with gunk and valves without? Well, those look like intake valves. You aren't seeing the inside of the engine, the piston heads, the rings, the top of the cylinder. I'm looking at what appears to be an oil consumption issue on mine, and nothing short of soaking the heads with a solvent and then vacuuming it out made a difference. And it did seem to help a little. And guess what? After a couple of hundred miles, the buildup had started to return. So I have more work ahead of me. (I have been using a USB borescope on a phone to examine my cylinders.)
I would not bother with adding more stuff to the gas (except I would not run anything but gas from a "Top Tier" station (google it) until you take care of a bunch of other things first. These would include:
- Adjusting the valve lash to specifications.
- Depending on the age of the EGR valve, it probably is time to replace it.
- Ensure that your air pressure is to specifications. Higher pressures can lead to a blowout especially with older tires.
- If you do not have the RE-92 tires on the car, accept that your mileage will be lower. If the tires are wider (for example, stock is 165/65r14; some people have put 175/65r14 or 185/60r14 on) you will see more of a drop. I would just accept this until it is time to change tires - the savings in gas isn't worth it.
The first two above made a big difference for me.