Gas mileage getting worse by the week

[91LTDProject]

This is really starting to piss me off. Back in August/September, the 91 was getting over 500km/tank with my driving habits, which are not that easy on the throttle. The gaslight would come on usually between 380km and 410km. Well, since then, the mileage has gotten progressivly worse. I checked the O2 sensors to see if they were functioning properly, and one was dead completly, and one was kinda sorta working, so I replaced them both. One of my wires started arcing, so I replaced them, and swapped my platinum NGK's out for NGK coppers. It has a newer cap and rotor, I routinlely run a carbon cleaning solution through the intake every 6 months. Changed fuel filter about 3 months ago when I discovered it leaking and thought that was the cause of my mileage, but it didnt help. Cant find a gasleak anywhere, and from the readings I took with my friends handheld infared temp gun, the cats are not clogged. I'm getting like 380km to 65L of gas, or 236mi to roughly 17gallons or 13.8mpg. When I first put the car on the road, with its orginal plugs, wires, cap and rotor, lopo upper, and 14 years of crap buildup in the engine, I averaged 19mpg city/highway. I'm out of ideas about where to look, and its starting to get ridiculous to drive this thing. Any suggestions on any sensors to check out. I know the fuel pressure regulators are common, but when the one went on my ranger, it chugged black smoke, and mine dosent smoke at all, though it has been failing the e-tests i run on it occasionally to check everything. Hydrocarbons were about 15-20ppm below the limit of 52ppm, now there up around 75-80ppm. Any ideas on where to start lookin fo the problem.

[mrltd]

ECT, ACT, MAP. Check wiring and vacuum to the map, and wiring for the others. The ECT and ACT are cheap, and go bad fairly often.\

my old 91 had the same issues. 19mpg no matter what. I hated it.

[91LTDProject]

Sorry man, total noob question, but could you define ect and act. also, what kind of numbers should I be lookin for at the map sensor, wiring and vacum, and what kind of numbers on the other sensors wiring. I know the map sensors are stupid amounts of money for these. The location of the ect and act would also be helpful.

[gadget73]

Engine Coolant Temp sensor and Air Charge Temperature sensor. ECT is located in the fitting on the front passenger corner of the lower intake where the heater hose hooks up. 2 wire connector. ACT is on the driver's side lower intake manifold just behind the #5 injector, also a 2 wire connector. You can meter them with a voltmeter to see if they're working properly. Charts with correct values here:

http://fordfuelinjection.com/?p=10

Definitely +1 on check the vacuum. Vacuum leaks make the engine run rich, which results in crap mileage and high HC levels. 17" or higher at the manifold is what the manual specifies, higher is better. Hook up the vacuum gauge (not to the MAP sensor line, since that has to be connected or the car runs like poo) and start pinching stuff off with needle nose pliers. If you see the gauge jump, you found a leak. Common leak points are the ATC thermal blower lockout, the "soup can", the parking brake release valve under the dash (never seen a good one of these actually), and the plastic lines over to the smog control stuff on the fender. Usually none of those are big enough leaks to cause major running problems, but they are enough to cause more fuel to be used.

[Heretic]

Not meaning to hijack, but I have a question about the parking break release valve - my 87 has a suddle hiss when it's in park, but when I select anything else (R,N,D, Etc) the hiss goes away. Is this valve what is causing this leak?

[91LTDProject]

I checked everything mrltd and gadget suggested and compared all my findings with the charts on www.fordfuelinjection.com and with the numbers off the graphing scanner. First of all, the vacum off the manifold was 20", above the minimum spec of 17" gadget said. At 20" vacum, the MAP was operating at 103hz, while spec is 104hz. At 0" vacum, it was reading 157hz, spec is 159hz. I have a handheld infa-red temp gun with a laser on it at work. With it aimed at the base of the ECT sensor, with the engine warm I had 211*F and a voltage reading of .66 to .68. The voltage spec for 212*F is .47V and the resistance measured across the terminals was 2.58K ohms, while the spec is 2.07K ohms. Is this far enough out of spec to be a problem? Readings off the ACT were 3.57K ohms resistance acoss the terminals at 180*F, Spec is 3.84K ohms at 176*F. At 160*F, there was 1.03V out the plug, spec is 1.04V at 157*F. So what do you guys think, ECT? Its the furthest out of spec. New ones over $50 canadian, which is kinda crap, and a new ACT is $25. New MAP is over $150. I didnt go to far into looking for any vacum leaks because the pressure was good. Any thoughts?

[mrltd]

The ECT and ACT look to be the problem, those are way out of spec.

[91LTDProject]

Ya, I figured I would have to replace the ect cause the specs are pretty far out, didnt think they were too out of spec on the act though. Shop foreman said it wasnt far enough out to worry about replacing it. But probably change it anyways. What are the sensors worth to you guys in the states? seemed kinda pricey to me.

[mrltd]

ACT is 11-28 bucks
ECT is 17-44 bucks

From partsamerica.com (I am aware you are in Canadia...)


And I am aware that I spelled Canada that way.

[91LTDProject]

Haha canadia. Thats quite a big difference in prices. I'm just gonna order them from our supplier. Will update if mileage improves/stays the same. thanks guys.

[mrltd]

Start with the cheaper one. IIRC the ACT has more of a chance to get messed up/go bad.

[91LTDProject]

Went back to work and tested the fuel pressure regulator and it seems like its working fine. Didnt have much time today but I decided to check the compression on the rear cylinder on the left bank. 100psi with the motor hot and the throttle wide open. Looks like the 91 needs a head rebuild. YAY. Still got to do the rest, but I get the general feeling it aint gonna be too great.

[gadget73]

You can sometimes fix the ACT by just pulling it, and spraying it with carb cleaner to remove the gunk from it. It might let you get a more accurate reading. Try that first before replacing it.

Heretic: yes, sounds like the parking brake release valve leaking in park.

[91LTDProject]

Ya, I've done that, then filled the tank back up with gas. So far no improvement. I think its gonna come down the the low compression. Heads have never been off. We dont have a leak down tester at work which kinda sucks.

[gadget73]

Are all the cylinders that low? Does the compression come up if you put a teaspoon of oil in the cylinder prior to testing? If the compression comes up with the oil, you have bad rings. Even with one weak cylinder, at 20" of vacuum (is it steady? steady usually indicates good valves) it should get in the mid teens on the highway. I run 100 psi on all 8 cylinders and 15-16" of vacuum and it does a steady 16 mpg on the highway at 70 mph. I do get like 9 around town because I have a heavy foot, but if I was nicer, I could probably do 12-13 without much trouble. If the motor is tired, it might simply be as good as it will be. If one of your other cars is in poor shape but has a good motor, I say pop the motor, rebuild it with better heads, and go. Or this is a good excuse to rebuild/upgrade the motor in the 91 while driving the other car.