Hi all,

I have a white '88 5-speed wagon with the 3S-FE engine and 159,500 miles on the clock. Recently, it has begun to idle roughly. It will "idle" at about 900 RPM and then sputter and fluctuate between 400 - 700 RPM then go back up to 900 RPM for a second or so and then go back to sputtering. It has never actually died, although it comes really close. I have also experienced a very noticeable drop in fuel economy. I filled the gas tank a couple of days ago and added Gumout Fuel Injector Cleaner. I then put on 324 almost entirely highway cruising miles on it and then refilled the tank to find that the car consumed 13.89 gallons (@23 MPG)of regular 87 octane. Friends and family tell me that the fuel filter is the culprit, so I went out to AutoZone today and picked up a newfuel filter and grabbed a new air filter while I was at it, since I haven't changed it since buying the car a year ago. Anyway, here are my questions:

1. Could the fact that I had added the Gumout fuel injector cleaner have anything to do with the dismal mileage I got on that tank of gas?

2. Where exactly is the fuel filter on a 3S-FE 5-speed wagon? Can someone explain or point me towards a diagram that could be found online?

3. Will it be necessary to remove the fuel pump fuse and then crank the car to relieve fuel pressure before changing the fuel filter? Could anyone provide a brief step-by-step forthe fuel filter change?

I really appreciate any help that anyone can provide. Money is tight and I actually like getting my hands dirty, so I'd like to DIY the filter change, if possible. In fact I want to be as "hands-on" as possible with this car, so I really hope some of you can share your knowledge.

Thanks in Advance,
Nick

P.S. I have already changed the air filter, but it has had no effect on the idling program.

 

The poor fuel mileage is not related to the fuel filter. Clogged(ing) fuel filters will show up with little power or lean burn missingat high speeds/rpms/loads when fuel demand is at its greatest.
Doesn't hurt to change it. I don't know where it would be on an 88 but if it's where the rest are for earlier years it'll be DS between engine and inner fender. I'd buy a Haynes manual to see the removal procedure.

I don't know if an 88 has a Idle Air Control valve (IAC) but usually when people post about poor or rough idle the IAC needs to be cleaned. You can do a search on IAC cleaning, or look in the Haynes.

As to the poor mileage the air filter may have had more to do with that than anything. 23 mpg does not seem bad for the year as I get 27 on my wagon. Have you done a compression test?

 

In replacing air filter, hold the old filter up to a light bulb. Check for holes, unless the filter is very dirty it typically does not have any effect on performance. The same is true for the fuel filter. As Pedro states the effect would be in sluggish performance not erratic idle.

Have you check the engine for damaged, cracked or loose vacuum hoses, wiring, etc. The ISC valve controls engine idle by bypassing air around the throttle plate. There is a small valve inside the valve body that is controlled by an electric motor via the computer. Often the valve becomes coated with engine grim leading to poor operation.

To check the valve operation look for the diagnostic box located passengers side shock tower next to the master brake cylinder. Open the lid and look underneath to see the terminal layout.

To test it warm up engine, place transmission in neutral (engage parking brake). Jump the E1 and T connectors in the box with a short section of electrical wire. The idle speed should increase to 1000-1300 RPM and drop back down to normal after 5 seconds.

If not OK stop engine and find the ISC valve located beneath the large air intake of the throttle body. Remove the electrical plug and you will see three terminals in the body ISC1, B+ and ISC2. Use an ohmmeter to check the resistance between B+ (center terminal) and ISC1 (upper) and B+ and ISC2. The resistance between the center terminal (B+) and each of the other terminals should 16-17 ohms.

You might try cleaning the valve by spraying a little throttle body cleaner inside. Make sure the cleaner is safe for use with plastics as the valve is plastic. The valve does come off the throttle body.

The fuel filter is located under the engine air intake duct, which needs to be removed to provide access. Relieve fuel pressure by removing the 15A EFI fuse and cranking the engine for a few seconds. Take care when removing the plumping fittings not to strip their heads if they do not come out easy.

 

Thanks a bunch, guys! I have printed out your replies and will go do some investigating under the hood later this evening. I will post my findings and any resulting questions.

Also, Pedro, the 23 MPG was literally 99% highway cruising at 65-70. Last summer, I was easily getting high 20's in mixed driving and low 30's on the highway. So you don't think the Gumout additive had any effect on that mileage result?

Again, thank you all very, very much. If anyone else has any tips or hints, please chime in. Otherwise, like I said, I will post what I find.

Keep Motorin',
Nick