I was having a hard time pinpointing the root cause of my horrible MPG on my 98 MAX 5spd SE 170XXX miles, even when feathering my throttle. Replaced O2 sensors, cleaned IACV, Throttle body, replaced air & fuel filter, Engine coolaant temp sensor, Air Temp Sensor, cleaned MAF sensor...And stil, my MPG was crappy...I could even see my fuel gauge go down while driving.

However, I read about cleaning the EGR tube...So I did, but without completely removing the tube, as I didn't hack the bolt so I couldn't remove it. One thing I noticed after (half-a$$) cleaning the EGR tube is that the MPG improved slightly. But I did discard that route as didn't think the EGR was going to have a significant impact on the MPG.

After that, I started to see that there was oil seeping out of my valve cover gaskets, but mostly, from the upper intake plenum. I checked the codes and there was a P400 code (EGR HI/Low flow)...

So I decided to go all the way, gathered some tools and some courage and prepared myself to tackle this once and for all on a "nice" 110 degrees summer saturday in Mexicali, Mexico.

I removed the EGR tube, this time I did the bolt hack, and removed the tube, the EGR BPT valve (Which had the hose that goes straight to the EGR valve cut in half, so I replaced it), I removed the EGR valve itself and cleaned it out good. There was a hose with a metallic section that was so blocked I had to use a wire an lots of carb cleaner. Also, cleaned the one way valve good(Don't know the actual name of this part, but on my 98 max SE, it was black on one side, white on the other).

While I was there, cleaned the TB and IAC valve again, as there was some gunk build up.

Then I resetted the ECU, adjusted the idle , and after a 100 miles or so, my MPG started improved...I don't have the actual MPG numbers, but I can say it definetly improved. And also the response of the car is better , I used to have some hessitation when WOT or when accelerating around 2500 RPM, but that is gone.

Hope this helps somebody, really, CLEAN YOUR EGR system, is worth the hassle, do it good, you won't regret it.

 

BTW, the oil seeping thru upper intake plenum & valve cover gaskets is gone also...I guess by having the EGR malfunctioning, was creating some "exhaust gas & oil blow by pressure buildup" to the plenum and valve covers, and also the hessitation.