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Poor Idle HELP!


steelhorse9859

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Hello,

This is my first post and am really looking to get some help I apologize if this isn't the right place.

I have been having an issue getting my 88 Bronco ii 5 Speed Manual to idle where it is supposed to. What happens is that when the engine is cold started the engine RPM will sit at about 1100 RPM for a few moments then it will slowly drop lower and lower. When the idle RPM levels out it stumbles around 500-600 and sometimes at startup it will stall out and die with the idle drop. The engine is supposed to sit right around 850 at idle as the 5 speed if it was an automatic it would be around 800. I am about to start pulling my hair out trying to figure out what to do to get this under control. I am able to drive the truck around and when it is in gear and moving their doesn't seem to be a complete loss of power. If I pump the brakes the idle will rise to where it is supposed to be, but once I stop doing so it will drop down to 750 and settle around 500-550. I am willing to post a video of what it is doing and link it if need be.

I want to include all the things that have been replaced so far in the truck. Now before I bought the truck (which was Jan 1, 2017) both fuel pumps (inline and external) and filler neck was replaced. I am not sure about the fuel filter.

Trying to figure out my issue and having a couple other ones I have replaced the following since Jan 1 2017.
New Spark Plugs (all where gaped to specs found in the service manual
New Plug Wires
New Ignition Coil
New Thermostat (190degree F)
New MAP Sensor
New Throttle Position Sensor
New Idle Air Control Valve (IAC Valve) - The most common thing I see to replace on the forums
New Coolant Temperature Sensor
New Air Charge Sensor
New PCV Valve
New Power Brake Booster
I also removed the oil sender unit and installed a mechanical gauge to monitor oil pressure instead of having a low oil pressure light which would only illuminate once the oil pressure falls below 5psi.

Any help here is so appreciated! I have double and triple checked the IAC valve and do not believe that my replacement is faulty.

P.S. There is no EGR valve on this model. If there is it is really hidden somewhere that might not make sense.


Thank you!
 


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Spott

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The '88 did not have the EGR valve.

Pumping the brakes to affect idle is odd. However, the brakes and idle are both affected by vacuum, so my first suggestion would be to test for vacuum leaks and inspect the vacuum system.

Get a can of starting fluid, and, with the engine idling, spray it a little around the vacuum hoses in the engine bay. If there's a leak, the spray will get drawn into the leak and the engine will rev up a bit. This should help locate any potential vacuum leaks.

Then, take the vacuum hoses off and inspect all the hoses, fittings, and manifolds. Also double-check that power brake booster you replaced, just in case.

If you find anything odd, let us know.
 

steelhorse9859

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The '88 did not have the EGR valve.

Pumping the brakes to affect idle is odd. However, the brakes and idle are both affected by vacuum, so my first suggestion would be to test for vacuum leaks and inspect the vacuum system.

Get a can of starting fluid, and, with the engine idling, spray it a little around the vacuum hoses in the engine bay. If there's a leak, the spray will get drawn into the leak and the engine will rev up a bit. This should help locate any potential vacuum leaks.

Then, take the vacuum hoses off and inspect all the hoses, fittings, and manifolds. Also double-check that power brake booster you replaced, just in case.

If you find anything odd, let us know.


Thank you very much.

It is really odd that it would do that too, I replaced the power brake booster because the old one was the stock one and had a leak on the brake lever side. (very audible)

I have been scouring the vacuum lines for leaks and have been coming up short every time. I will try starting fluid and post my findings.

As far as inspect manifolds, what would you suspect as a higher likely hood over others? Also what should I be looking into as far as the power brake booster goes?

Take it there is virtually no rust on this thing and just has some old hoses and lines so I believe that it may very well be a vacuum leak it has just been a nightmare to find out where.
 

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The manifolds are all connected together, so a leak will have the same effect regardless of where it is.

For the brake booster, try disconnecting the vacuum hose that runs to it and sealing the hose. Then see if your idle works correctly. This removes the booster from the vacuum system, to help isolate the cause of the problem.
 

steelhorse9859

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The manifolds are all connected together, so a leak will have the same effect regardless of where it is.

For the brake booster, try disconnecting the vacuum hose that runs to it and sealing the hose. Then see if your idle works correctly. This removes the booster from the vacuum system, to help isolate the cause of the problem.
I will give that a try and see where I end up and I'll post my findings.
 

steelhorse9859

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The manifolds are all connected together, so a leak will have the same effect regardless of where it is.

For the brake booster, try disconnecting the vacuum hose that runs to it and sealing the hose. Then see if your idle works correctly. This removes the booster from the vacuum system, to help isolate the cause of the problem.
I've went ahead and tried plugging the brake booster and it did not effect the idle so it's safe to say that the booster is not the issue. I also did what you said and tried starter fluid around all the vacuum lines and manifolds to no prevail. The only place I got anywhere was around the base of the air box, but that's because it was sulking it directly into the intake. I have posted a video of what it's doing on YouTube.

https://youtu.be/dYV8CizppVo

Thank you for your help.

Sent from my VS990 using Tapatalk
 

steelhorse9859

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I pulled the vacuum line off of the fuel pressure regulator to see if I could tell if my regulator was bad andno fuel was in the line. I also turned the key to prime the fuel rail and nothing leaked out of the vacuum side of the regulator. I am still quite stumped. I am prepared to just replace it to eliminate it from the list of potential things that it could be.
 

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Try using a propane torch to blow propane over the engine. If the engine speed increases, then you have a leak where ever the torch nozzle is. Also, do these trucks use vacuum for the heater controls or 4x4 like some fords do?
 
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wildbill23c

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Try using a propane torch to blow propane over the engine. If the engine speed increases, then you have a leak where ever the torch nozzle is. Also, do these trucks use vacuum for the heater controls or 4x4 like some fords do?
No vacuum control for the heater controls, still a cable. No vacuum for the 4x4 system either, its electric or manual.

The idle on my 88 B2 does the same thing. Got a vacuum leak somewhere just haven't had time to trace it. Pretty sure its vacuum related as I have issues with the cruise control working properly as well whenever I have the issue with the idle dropping and trying to die.
 

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No vacuum for the 4x4 system either, its electric or manual.
I thought that some models used vacuum to automatically engage/disengage the front hubs, and that this was a known failure point?
 

wildbill23c

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I thought that some models used vacuum to automatically engage/disengage the front hubs, and that this was a known failure point?
On the newer models they sometimes do, but most were electric or manual. Could be wrong though, I know the mid 90's explorers had the vacuum actuated front axle...no hubs just an actuator those things were good for failing...Jeeps had the same issue.
 

steelhorse9859

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On the newer models they sometimes do, but most were electric or manual. Could be wrong though, I know the mid 90's explorers had the vacuum actuated front axle...no hubs just an actuator those things were good for failing...Jeeps had the same issue.
The 4x4 is through manual locking hubs. I checked all over for vacuum leaks. Propane, starter fluid, seafoam and carb cleaner. There is definitely not a vacuum leak.

I replaced all 6 injectors and the fuel pressure regulator yesterday and it seems to be running a lot better. I am now faced with an issue of my engine temp sensor not giving me an accurate reading. I let it idle after the replacement to try and drive any air out of the fuel lines for about 20 minutes and then drove it 10 miles or so and the temp gauge did not get up into the normal range it stayed right below the normal line after the C on the scale.

This leads me to believe that even though the engine is warmed up it is not telling the computer the right temperature which also effects the idle.

I don't know what I would need to do in order to test to see if it is bad or not. It isn't exactly in an easy to access (remove the thermostat housing and possibly a heater hose) Which sucks because it was one of the things on my list that I replaced.

Is there some sort of regulator or something that sends a signal to the dash/computer? The original sensor was the one from the factory. I planning on hooking up my mechanical gauge to it by splicing the upper rad hose, but not anytime soon.
 

wildbill23c

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Temp sensor may be faulty or your thermostat may be faulty and stuck open. Just because you replaced it doesn't mean its good. Are your heater and radiator hoses both warm after a long drive? If so the temp sensor is probably faulty...not that they or that gauge is accurate in the first place, they are nothing more than glorified idiot lights. I have not done it yet on my 88 B2 but planning on installing a 3 gauge cluster that gives actual temp reading, oil pressure, and voltage.

I'm not sure how you test the temp sensor, probably a certain number of ohms at certain temperatures but I'm not sure what those readings should be.

Faulty temp sensor could be your problem, if the computer thinks the engine is cold, or hot it can change your idle speed...not sure it would increase the idle speed if its hot though. I believe that if you have the AC on the computer adjusts the idle higher due to the AC being on, but I don't think it increases the idle if the engine is running too hot.
 
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steelhorse9859

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Temp sensor may be faulty or your thermostat may be faulty and stuck open. Just because you replaced it doesn't mean its good. Are your heater and radiator hoses both warm after a long drive? If so the temp sensor is probably faulty...not that they or that gauge is accurate in the first place, they are nothing more than glorified idiot lights. I have not done it yet on my 88 B2 but planning on installing a 3 gauge cluster that gives actual temp reading, oil pressure, and voltage.

I'm not sure how you test the temp sensor, probably a certain number of ohms at certain temperatures but I'm not sure what those readings should be.

Faulty temp sensor could be your problem, if the computer thinks the engine is cold, or hot it can change your idle speed...not sure it would increase the idle speed if its hot though. I believe that if you have the AC on the computer adjusts the idle higher due to the AC being on, but I don't think it increases the idle if the engine is running too hot.

Yes the heater hoses and the radiator hoses are warm after a long drive. It doesn't take that long for them to get to a consistent temp. This is true that they may not exactly be accurate, but since it is an analog two wire resistance system it basically sends certain resistance to the computer to tell it whether or not its 0 degrees F or 210 degrees F or something else and along with the O2 sensors and Air Charge Temp Sensor they tell the computer what the fuel injection ratio should be along with the timing. If the thermostat was stuck open wouldn't it still give a reading on the gauge. I am only getting up to the line past the C not all the way into the normal range?

I'm definitely not dealing with an engine that is getting too hot, but I see what you mean with it being potentially a faulty sensor. I am pretty sure I have a warranty through AutoZone for the sensor and the thermostat. I might as well replace them both again since I will have to take the housing off the thermostat to get a wrench around the temp sensor.

I guess I was just wondering if there is a module that regulates the gauges on the dash. Like in the steering column or under the dash somewhere or hidden behind the gauges that may get disconnected? I'm grasping at straws since I have been at this for some time.



I have the 3 gauge cluster in my B2 and it is pretty nice, I used an add a fuse to direct it into my fuse box. For the manual temp gauge I am planning on getting a replacement upper radiator hose or lower (whichever the length of the gauge wire permits) and cutting it in half and fitting some high temp plumbing with some hose clamps in the middle and fitting the temp thermometer into the pipe. I based it off one that's marketed to do the same thing except will cost a lot less. I didn't reinstall the oil pressure sender since it is literally just a dummy light and the voltage gauge I just used a separate ground and wired it into the same power source as the lights for the gauges. Looks pretty nice and works out really well.
 

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The temperature sensor for the dash gauge is just wired straight to the dashboard. There's a completely separate sensor that the computer uses to read engine temp. FYI, there's only a couple possible auxiliary modules in the Gen 1 RBVs, such as electronic 4WD, or cruise control. Almost everything is direct-wired or goes to the main computer.

If your thermostat is stuck open, then the radiator will always be in the loop cooling the engine, and the engine will never get up to operating temperature. In that case the sensor would be working correctly, indicating that there's a problem.
 


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