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High Idle after throttle body rebuild

  #1  
Old 09-02-2017, 07:25 PM
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Default High Idle after throttle body rebuild

Hey, just finished a rebuild/cleaning of throttle body for my '88 blazer, watched it spray gas for a bit after and it looks great. Only problem is in park the idle is pretty high and kinda sounds like ****. Once I pop it in gear it sounds good and runs better then it did before. Assuming I just made some stupid mistake and hoping someone else has done it before and might recognize the issue.

On a side note, noticed a few odd things while tearing it all apart and wondering if anyone had some thoughts on them. First thing was air filter had gas in it, then noticed wires that go to one of the injectors had burn marks and partially melted, and also had some carbon/charring on inside of throttle body plate on same side as melted wires.
 
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Old 09-08-2017, 04:56 PM
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What was the reason for the rebuild? A lean condition? Could explain the backfiring in idle.

What comes to my mind with high idle with a TBI:

- Throttle linkage! Make sure that from the pedal onwards all the bowden cable stuff is in it's proper place.
- Make sure it fully releases the throttle and that the throttle plates are fully closed. Eventually unhook the throttle linkage you will immediately see if the rpm drops to normal level.
- IAC (not sure if the 88 has it.) The pintle of the stepper motor can become covered in carbon and get dirty and sticky.
- Simply the gasket under the foot of the TBI - If it pulls air and MAP falls it will add fuel which rises idle rpm. Mostly with a tendency to hunt for a stable rpm.

For the wires!
Sounds like it had backfired.
Have that fixed ASAP. Even mini arching against the body of the TBI or against each other could start a fire. Best would be to replace the wiring harness to the injectors.

Have you done a complete rebuild with all the gaskets, o-rings and fuel pressure regulator membrane, or just disassembled and cleaned?

Some pics while rebuilding my TBI
https://blazerforum.com/forum/builds...t-92047/page3/

In the image with the TBI spread over the white sheet of paper the IAC stepper motor with pintle is in the top above the TBI body. The hex is about 1" or even bigger.
 

Last edited by error_401; 09-08-2017 at 05:08 PM.
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Old 09-15-2017, 03:07 PM
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The reason for rebuild was actually running rich, periodically bogging down between 25-40 mph, and the gas in the air filter. I did do the complete rebuilds with all the gaskets, membrane, etc. That is a very good point on replacing the wiring harness, had not thought of that and the wire are definitely more exposed then they typically would be due to the melting. As for the IAC, I removed it during cleaning rebuild and it looked in pretty good condition. I did notice that in drive and giving no gas it will get up to 15 mph or so, seemed a little fast. If your idle rpm is to high will the IAC give issues due to not enough air and make it run bad in Park and neutral? Could these be caused my TPS sensor when I put it back on?
 
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Old 09-16-2017, 06:07 AM
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Should not be the case.

The IAC on these engines with high vacuum is pretty strong device. Up to 15 mph could be possible - i guess automatic transmission. And the IAC valve is actually the air access in idle control.

The IAC will rise the rpm whenever something occurs which needs a higher idle, such as A/C kicking in, cold start, or being put into gear. Especially when cold I would expect to see something like 15 mph on a flat stretch. I like that on mine in traffic jams. Just put it in second gear (automatic) and let it roll. On flat portions of road it will do 15 mph in idle.

TPS when put in correctly - and that should be the case, because of the screw location and the shaft, is self learning for the full open and full closed. As long as the throttle plates are closing all the way mechanically the ECU will adapt for minor changes. On the TBI 220 no learning - it will do it by itself.

IAC troubles normally causes an erratic idle or stumble. Sometimes even stall when you put it into gear or the A/C kicks in when in gear. So does not sound like that is the problem.

Check all vacuum lines for leaks. If you have a leakdown (vacuum) tester you can test all the lines. Especially look for the two or three rubber knees in the lines to and from the MAP sensor. Any leak into the manifold may cause this.

How about the rich condition? Did it go away after the rebuild?
Another thing that comes to my mind is the injectors themselves.
Remove the air filter and have somebody turn the ignition to "ON" but do not start. The fuel pump should run for a couple of seconds pressurizing the system. Check for leaks! Careful, we are talking fuel here. If one or both injectors drip fuel or if fuel is leaking somewhere in or around the TBI that is not good.

Did you remove the injectors? If yes have you put them together with new o-rings?

One more thing to check would be injection timing. I would just go and check that ignition is doing the correct job
 

Last edited by error_401; 09-16-2017 at 06:14 AM.
  #5  
Old 09-16-2017, 09:20 PM
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Rich condition got a little better with new distributor and the throttle body rebuild. The spray from injectors looks perfect, no drips and a very fine mist, although there is a bit of a high pitch hiss coming from injectors that you can hear with air filter assembly on,just assumed that was normal. Replaced the map/throttle body vacuum line today as it was starting to split at the fitting, seemed to have no affect. Also did seafoam in gas and oil before giving it an oil change including oil filter and air filter. Added some lucas oil additive after change. I have been watching the fuel and fuel return line into throttle body for leaks pretty carefully after the rebuild, thinking of switching to the braided steel lines pretty soon but no leaks that i can tell. Still have a hell of a smoke cloud on start and a bit rich judging by the smell coming in the back window when down. Also, still a bit of gas in the air filter.
 
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Old 09-17-2017, 02:40 AM
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A bit of a hiss sounds normal for an idle condition. The engine still pulls quite a bit of air through the small holes in the throttle plate and the idle valve.

The fuel issue sounds like a leak somewhere in the TBI. Funny that it puddles in your air filter.

Have you changed the large and the small o-ring on the injectors?
 
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