The Search for the Source of an Idle Surge, Pt. II

"Urambo Tauro" (urambotauro)
03/18/2016 at 19:01 • Filed to: wrenching, beater build, blog, murlequin

After all that nonsense with the !!!error: Indecipherable SUB-paragraph formatting!!! ’s distributor, no discernible progress had been made toward resolving the surging idle. It had to be something else. This is part two of that adventure.

Back to square one, I guess. What could it be? I realized that I had missed some spots when looking for vacuum leaks, so I ran the engine again and sprayed around the brake booster... still nothing. I removed and cleaned up the IAC, likewise with the EGR... still no change.

I started to wonder if there was perhaps a vacuum restriction somewhere, perhaps within the throttle body. So I bought a rebuild kit and took the TB apart to clean it. Pretty much everything was covered in dark grime except for the injectors and the throttle plates underneath them, which operated nice and smoothly.

I scrubbed, scraped, and washed the TBI components, removing two decades and 288,000 miles of gunk. I did not find any blockages, but there were some restrictions. The worst buildup was near the opening of the passage that connects to the PCV valve; it looked like it was covering about half of the opening.

There was also some buildup in the IAC passages; perhaps not enough to cause much of a restriction, though. The bottom half of the fuel pressure regulator had a lot of rust and crud inside. I replaced the diaphragm, but re-used the spring after cleaning it up. The outlet nut for the fuel return line had part of its gasket hanging in the way, causing another partial restriction.

LEFT: The rusty bottom of the fuel pressure regulator diaphragm. RIGHT: A damaged injector O-ring and its backup washer.

When I removed the injectors, I found that the O-rings were damaged, as if someone had installed them dry, and the backup washers were on top of (not underneath) the upper injector O-rings. More reasons not to just put it back together the way I found it.

I noticed that the injectors were not OEM. The previous owner didn’t mention them, but perhaps that had been another attempt to fix the idle. The injectors appear to be new and have “RIN-1001" printed on the top. I punched the number into Google, but there’s not much info to be found on them, not even a brand name. Weird. Oddly, according to the notches inside the bowls, one of the injectors was installed backwards, so this was my chance to turn that around.

The throttle body, fully cleaned and assembled. I was so excited to clean it up that I forgot to take a “before” picture. You shoulda seen it...

Before putting the freshly-rebuilt TBI unit in place, I double-checked the vacuum lines for blockages, but they all seemed fine. But to my surprise, I found a crack in the nylon EVAP line, hiding behind the alternator. You know, I never did spray there when I was looking for vacuum leaks. So I threw a quick patch of electrical tape on it, at least for now.

I put everything back together and took it for a test drive. SUCCESS! I was rewarded with a smooth steady idle, even when standing still. But just out of curiosity...

I unwrapped the patch that I had put on the EVAP line and took it for another drive. The surging idle returned immediately. Are you kidding me? After spraying around multiple times for vacuum leaks, not only did this one escaped my notice, it was the source of the problem all along! Lesson learned.

But I’m kinda glad I rebuilt the TB anyway. It didn’t cost much, and should now be good for years to come.

DISCUSSION (4)

Birddog > Urambo Tauro
03/18/2016 at 20:43

That’s actually a pretty lucky find. Without a smoke machine that would be a hell of a leak to track down.

It is probably for the best you did the rebuild, that gunk could have caused issues later on.

Urambo Tauro > Birddog
03/18/2016 at 21:00

I agree.

I’ll admit that part of me is trying to justify the unnecessary spending. However, from what I saw, I really do believe that taking care of these items now will prevent future trouble. Besides, I’d rather do the work now than on the side of the road, in the rain.

uofime-2 > Urambo Tauro
03/21/2016 at 16:00

Does it run better (beyoned the surging idle) now?

After all the sketchiness you found and corrected I’d assume it would!

Urambo Tauro > uofime-2
03/21/2016 at 16:18

Idle is now steady. I hope it stays that way.