I do have a timing light for older cars. Isn't it the same way to check Bronco? Thanks!

 

get yourself a chiltons or Haynes manual. also a google search will show you how to properly check the timing on the bronco.

 

Short answer: No

Just like the old days you had to remove the vacuum line from the distributor (vacuum advance canister) to set the base timing. This is done by removing the SPOUT plug to disconnect the computer control of the timing.

 

But I don't need to disconnect anything just to check the timing, right? Only if I find the timing is off and needs to be adjusted, then I have to set the base timing right by disconnecting those, right? Just want to make sure. Thanks!

 

You did not read my previous reply. You have to remove the SPOUT plug to check the base timing. That's how the factory 10 degrees BTDC is measured.

There is no other way to "check the timing". It will vary under various operating conditions.

 

OK, I just unplugged the SPOUT and checked the timing: it's about 5 to 6 degrees so it's off. I will adjust it later. Could this be the reason why I have this kind of oscillating low idle or there must still be something else? Thanks!

 

Now it's 10 degree. But that didn't solve the problem: low idle is still the same, oscillating between 1000 to 500 RPM...
Doesn't look like it's caused by timing.

 

well you have to get everything set correctly to then lead you down the correct path. so now we all know your timing is set correctly.

here are the instructions for properly setting your idle screw.
1983 Ford Bronco TSBs & FSAs (Recalls) for '83-96 Broncos & F150s picture | SuperMotors.net

once this is done, going thru your vacuum lines. heck even a bad intake or throttle body gasket could cause a vacuum leak.

 

Since that idle screw came with a locking sealant mark, I can restore it to the original location based on that mark. So it should be close to its original position now. I also found it doesn't make much difference once it's backed off enough.
I will use a manual vacuum pump to check all lines next. I guess I need to unplug each line and vacuum on it and check the meter. How long it should hold to be considered as good? Thanks!

 

That is not an idle set screw. It's a throttle stop screw. It's only purpose in life is to prevent the throttlebody butteryflies from closing too far then sticking in the bore. That's it. Nothing more beyond that. Put the screw back to match up to the thread locker, done.

Much difference in what?


In my experience several minutes. Be aware the vacuum circuit for the EVR will fail the vacuum test. In the closed position the EVR acts as a vent to atmosphere. This is normal. That was a $45 mistake I made a few years ago. Everything else should pass.

 

Then I don't know which one is idle screw. Your link only covers older model Broncos.

Since both IAC and TPS are new and nothing changed after I replaced them, I guess they are not the problem either.

I will check fuel pressure and vacuum next. Thanks!

 

I never posted a link for adjusting that screw. Yes that Ford procedure is long and drawn out and does have you adjusting it to some low value idle RPM with IAC disconnectex but I personally find little real value with it.

Since you can put yours back to the threadlocker line there is no sense doing stupid human tricks to find you still have the same issue.

 

I finally had a chance to check the fuel pressure: it's only about 30psi average and oscillates around that. So the fuel pump is bad (the fuel filter is new). I will replace the fuel pump next and let you know.

I know I have to drop the whole tank in order to reach the pump...

 

30 PSI is a bit low but could be your gauge accuracy.

Does the fuel PSI increase when removing the vacuum line to the fuel pressure regulator?

Does it increase when "blipping" the throttle?

 

Since it's oscillating around 30psi, it could be lower than 30 sometimes. That's why I didn't check anything else before I conclude it's pump problem. I also noticed the noise from the pump.