Okay I have researched the crap out of the problem i am having with my 1992 ford bronco but I cannot seem to nip this problem in the butt.

My truck has a fluctuating Idle only during one condition. When I first turn the bronco on in the morning after letting it sit for a night. When I fire it up it goes to high idle as normal. After 30 seconds it idles down to what looks like 850-900 rpm but then about 15 seconds after this the idle begins to fluctuate. It will rise to around 925rpm then drop to like 675rpm the rise back up to 925rpm. It will do this for about ten minutes if i let it sit there and idle. Then it will go away. If I turn the truck off, during this problem, then fire it back up the problem goes away. It will no happen again till I let it sit over night again.

I have removed the Air injection system. I have the heads plugged and the solenoids still plugged it but capped off. The throttle position sensor is new and has a good voltage sweep. Air intake temp sensor is new. I have tried three IAC valves; Original 2 piece ford, newer one piece from napa, and a newer one piece from ford. Tried 2 throttle bodies; original 1992, and the current one is from a 1996 351 e series van. All vacuum lines are new and i have exhausted the possibility of a vacuum leak.

The only thing I an deduce is that the computer wants to see the air injection system running during its rich warm up. This is a bit of a shot in the dark.

I have no running codes during the KOER but do have some memory codes. Most are to do with my e4od being on its last leg and having a shot torque converter but there is code 173 in the memory. This says HEGO shows system always rich. I have no check engine lights and no flashing OD light. Maybe This code is Linked to my problem.

Any help with this problem would be very helpful. I have not been able to solve this on my own.

Bronco Specifications:

5.8 e4od
Air injection system bypassed
1.7 roller rockers
msd coil and ford racing wires
custom y-pipe into 3 inch cat.
bbk shorty headers
Rebuilt E7TE heads with ARP head bolts.
New Sensors: TPS, IAT, HEGO, Coolant temp sensor, IAC Valve.
New rebuilt distributor. Timing at 10 BTDC

 

Clean the Throttle Body (follow the instructions on the TB cleaner can) and the Injectors, that will probably help you. Btw, rebuilt/reman distributors like to cause trouble, i would see that up.

 

Throttle body is as clean as new. Cleaned it when I put the new tps on. Problem was there with my original distributor. I had to replace it because screwed it up when I was putting my rebuilt heads on, I left it in the rain and it fried.

As far as the injectors go I have run sea foam through the gas tank. I guess if they were slightly clogged this could happen. Any way to tell if the injectors are failing while on the truck. Truck seems to be great just this one problem.

 

Code 173 (Fuel system at adaptive limits) - basically is telling you that the system is unable to compensate for the rich condition. With arbitrary parts and pieces of the emissions and Thermactor system disabled, disconnected or not functioning there is no way around this. All it will do is annoy you, shorten the life of your spark plugs, and eventually shorten the life of the rings. Emissions removal in bits and pieces on EEC-IV and newer engines simply doesn't help anything because the systems are so well integrated. The engine will run BETTER if everything is in working order.

 

I understand that removing emissions equipment can have detrimental effect. But I would not say that removing the air injection would cook my rings. Air injection happens after combustion. I remember reading that air is injected only during warm up to light up the cat and also on decell to burn up any raw gas. I don't think this should be causinng my problem anyway because it only happens intermittently.

Is there a certain code scanner we can get for our trucks to watch what the computers sensors and actuaries are doing? I know of tweecer or whatever it's called. Is there anything else.

 

did you get a faulty throttle position sensor?

 

No I did not. I replaced it just in case there was an issue. Tps has a good voltage sweep and a good ohm's sweep.

I was brain storming and I was thinking maybe my injectors are slightly clogged. Because this problem only happens on the first cold start. The engine could be bogging down and then revving up because the injectors are not spraying fully. I just figured this wasn't an issue because my truck drives great with no power problems.

 

okay to update this post I have gone through all my injectors and put new end caps on them and new O-rings. Also cleaned up a new upper intake manifold and put all new gaskets on across the whole upper plenum.

Today I bought a Vacuum pump and checked all hoses for vacuum leaks. Could find none. Everything held a 20 Inch of Hg. Vac.. I ran into a bit of a confusion on how to test the E.V.R actuating valve. My manual says that on the main vacuum inlet port to the E.V.R it should hold vacuum. This is not the case. The Vacuum bleeds through the valve. I tested this on my E.V.R and a brand new one. Both Bleed Vacuum on the inlet and exit ports.

Truck still runs great with great power. Took it wheeling a few times this summer. I love this thing! Just this pesky problem.

So now i move onto problem solving something else which is causing my cold idle fluctuation.

1) I'm going to look into the fuel pump. Its most likely the original and I am wondering if the fuel pressure is fluctuating from the pump. I do have a new F.P.R on there.

2) I know the bearings in my original power steering pump have seen better days. Maybe they cause my idle to hunt because the computer is trying to hold a steady idle and the bearing are causing greater resistance. Pump is a screamer.

3) I have had my engine wiring harness fixed in a couple of spots. Thinking of replacing it with a new one from another truck. Maybe the ignition suppressing wire, covered in tin foil going from the distributor to the computer, is compromised.

4) Going to look into if my 1.72 roller rockers are giving my trucks Computer a problem. Still researching this. I do have steady 20 in Hg. Vac. at idle so I dont think so.

Any other ideas for me to check out would be Great if anyone can help me out.

 

A faulty ACT or ECT sensor can cause cold idle issues that will not turn up in a KOEO or KOER test. To test these sensors, you can start with them at the ambient temperature and double check their resistance value. The next step is to warm them up to see if they are going through their full resistive range. I've heard guys talk about everything from hair dryers to putting 'em in the oven with the leads for their multi-meter run out the door to keep an eye on the resistance changes. Personally, I use a heat gun (paint stripping type). I have posted the resistance chart before but here it is again.

 

I replaced both of these sensors a while ago. It doesn't mean there are no problems though. I will Check the Resistance with the graph you attached.