[92BroncoS]

Just the other day on Wednesday my Bronco started misfiring while going to work, but just intermittently. On Friday I took all of my spark plugs out, cleaned them up, regapped them all and the truck ran great again no misfire. Saturday morning and it's back again....... So today I dived into it and replaced all my plugs, wires, and ignition coil. Plugs and wires only had 20k on them, but the ignition coil hasn't been changed since I owned it so I figured I would replace it. Again no luck truck is still misfiring. Doesn't seem to really do it at idle but does it quite consistently when I apply the throttle.

I have a new Bassani Y Pipe and Headers in my garage I've been waiting to put on so I said screw it and cut the exhaust off just to rule out my cats being clogged. While also removing the rest of my air injection system since my air pump has been bypassed for a few months now anyways. Again no change now the truck is just a lot louder and harder to hear the misfire! I don't think the issues are related but the truck has had a high idle since I've owned it which I'm not sure what that is either. I'll attach below a list of all I've replaced on the truck since I've owned it cause it's a lot... I've tried pulling codes on the truck KOEO and just get a 111 everytime, and now I had the battery disconnected while working on it today so I certainly won't get any. I've done so much reading on this the past few days and I'm just at a loss at this point. Any advice would be greatly appreciated!

 

[robbz28]

When you pulled the plugs did any of them look different than the others? Did you "read" them? I had a misfire when i first got mine and one of the injection wires was shorting out on the intake, little furry critter had nibbled on the insulation.

 

[92BroncoS]

All the plugs pretty much looked the same like this. I’ll look over my injector wiring later and see if I find any shorts in the harness. I have good fuel pressure to the rail, but wondering if maybe I have an injector issue? Truck has 217k on it and I don’t know of them ever being replaced. I have a set of the upgraded 4 hole injectors on order so I’ll be putting those in along with lower/upper intake gaskets this week which will hopefully help my high idle at least, and maybe my misfire? Truck does it just in park if I rev it just starts missing, and is so bad it’s not driveable.

 

[robbz28]

It makes me wonder if your coil is getting all the juice it needs to work. Like maybe a bad ground or some other wiring issue...or a vacuum leak.


Sent from my iPhone using Tapatalk

 

[92BroncoS]

I did put in a new coil yesterday but I should check the voltage it’s getting, I’ll check that. It definitely has a decent sized vacuum leak, the truck normally idles around 1000-1100 in park drive etc. I’ve checked for for vacuum leaks multiple times and the only things I think it can be at this point are the PCV valve, upper/lower intake gaskets, or my throttle body is worn out and extra air is getting by the blades. The other day I had the truck running and blocked off both throttle ports with plywood and it was actually idling where it should be it didn’t want to die. So I assume it’s obviously getting air from somewhere else.

 

[miesk5]

Yo 92BroncoS,

See my Vacuum leak test in post #11 incl jowens126 's HVAC Control Panel info & Mikey350's tests @ Help with dtc codes and idle
Excerpts;
When vacuum leaks are indicated, search out and correct the condition. Excess air leaking into the system will upset the fuel mixture and cause conditions such as rough idle, missing on acceleration, or burned valves. If the leak exists in an accessory unit, such as the power brake, the unit will not function correctly. Or Air Conditioning when in MAX mode may switch to Defrost.

&
"...If the engine still runs too fast with the throttle opening blocked, try taking off the IAC and blocking those openings.
What you have done is (supposedly) removed all the "normal" air intakes, and if the engine still runs, you have a vacuum leak.
You could then try leaving the air intakes (throttle and IAC) blocked and cap off the vacuum lines coming off the vacuum tree. (except the MAP line) If one of those makes a difference, investigate the leak.
Check the vacuum line from the vapor canister (on the left side of the throttle, when facing the throttle assembly) (cap it, on the outside chance that the solenoid has failed open) (if equipped)
Pull the EGR connection to the intake and plug or cover it. (if you have wide duct tape, use that to cover the hole)
If it still is running with the throttle blocked and the IAC and the lines from the vacuum tree capped, then you have a manifold or PCV line leaking. Plug the PCV line to the intake, then the vacuum line to the brake booster (if it has it's own fitting on the manifold)
Something is causing that high idle, and 95% of the time it is a vacuum leak."by Mikey350

 

[Tom S.]

When I bought my 96 4 years ago it had a miss. I did a full tune up, new distributor same thing. Turns out the #8 cyl injector went bad. Swapped it out with a junk yard one I paid 2.00 for years ago and its still in there running great.

 

[92BroncoS]

@miesk5 @Tom S. thank you for the suggestions I will look into all of these options. I had a few minutes free after work and started up the truck and guess what no miss…… after it ran for a minute or so then it started missing again. I also noticed my fuel vapor canister purge valve was making a clicking noise as the engine started missing. I shut the truck off and restarted the truck and again no clicking or miss…. @miesk5 i see you mentioned I could cap off the vacuum line from this solenoid to test it if it is sticking. I’m going to try that tonight. Any possibility that being stuck open could be causing my misfire or strange coincidence?

 

[92BroncoS]

So scratch that idea, just unplugged the fuel vapor purge valve from the throttle body and capped the port on the TB. When the truck first starts for the first minute or less I can hold the throttle around 2k and it runs perfect no miss. Then very shortly after it starts and is consistent.

 

[bmad01]

I don't know how long you've owned this rig, but let me tell you about my 93. I bought the 93 7 years ago and dumped a ton of cash into it. The front suspension was garbage, that got fixed first. The trans was toast, but before I could get that rebuilt the rear end blew up and needed a rebuild. Got that done and moved onto the trans rebuild. Got the trans done and feeling pretty good. I only drive the 93 on the weekends if it's nice. No snow or rain driving. Recently it started coughing like yours is doing. It would fire right up, but when you stepped on the gas it would freak out! It had good fuel pressure at the rail, but it was starving for fuel. It didn't have any codes either. I was telling my brother (a mechanic) about it. He asked me if I'd ever replaced the fuel filter on the frame under the drivers seat. Of course I haven't done that! I've only put approximately 25,000 miles on it since 2014 so everything I replace is new to this rig. I went to Oreilys and got a Wiss fuel filter and brought it home. It was then I discovered I needed a "release tool" to take the filter off. Ford has some funky one way circular snap clips on either side of the filter. They fill up with road grime and dirt making it almost impossible to take apart. After 3 hours and a lot of swearing I had the new filter installed. I used a paint marker and wrote the current mileage on the filter. They are only good for around 60,000mi. I fired the rig up and it was like it never had a problem. I took it out for a test drive and all was well! Still running good, till the next thing...

 

[92BroncoS]

I don't know how long you've owned this rig, but let me tell you about my 93. I bought the 93 7 years ago and dumped a ton of cash into it. The front suspension was garbage, that got fixed first. The trans was toast, but before I could get that rebuilt the rear end blew up and needed a rebuild. Got that done and moved onto the trans rebuild. Got the trans done and feeling pretty good. I only drive the 93 on the weekends if it's nice. No snow or rain driving. Recently it started coughing like yours is doing. It would fire right up, but when you stepped on the gas it would freak out! It had good fuel pressure at the rail, but it was starving for fuel. It didn't have any codes either. I was telling my brother (a mechanic) about it. He asked me if I'd ever replaced the fuel filter on the frame under the drivers seat. Of course I haven't done that! I've only put approximately 25,000 miles on it since 2014 so everything I replace is new to this rig. I went to Oreilys and got a Wiss fuel filter and brought it home. It was then I discovered I needed a "release tool" to take the filter off. Ford has some funky one way circular snap clips on either side of the filter. They fill up with road grime and dirt making it almost impossible to take apart. After 3 hours and a lot of swearing I had the new filter installed. I used a paint marker and wrote the current mileage on the filter. They are only good for around 60,000mi. I fired the rig up and it was like it never had a problem. I took it out for a test drive and all was well! Still running good, till the next thing...

I did replace the fuel filter about 13k miles ago which I think the interval is every 12k miles so I suppose I am due for a change. At the very least I should replace it as maintenance and who knows maybe there was shit in the tank I didn’t know about and it clogged up.

***I’ve actually seen multiple different intervals for changes so I’m not sure if the 12,000 is correct. That seems kind of soon and I’m assuming if I did have crap in the tank it would’ve clogged a while ago when I first replaced the filter.

 

[BroncoJoe19]

To rule in or out valve problems you might try this

Quick Tech: How to Read a Vacuum Gauge to Pinpoint Engine Problems - OnAllCylinders

A vacuum gauge is an invaluable resource for tracking down engine issues. The key is understanding what your gauge is trying to tell you. We’ve put together this quick-guide to …

www.onallcylinders.com

 

[92BroncoS]

I’m pretty sure I have a vacuum gauge in the garage somewhere I’ll definitely try that tomorrow, I should have a long time ago.

 

[Tireguy]

As written above valves are a possible cause of a misfire and a vacuum gauge will help you but lets hope not. OBD1 takes a little more work to pinpoint issues but I think you can still narrow it down manually. If the vehicle is misfiring at idle you can disconnect the injectors one by one and monitor the idle plugging back in as you go. The vehicle should run rougher when disconnected if not the cylinder that doesn't change could be your problem now you need to figure if its air, fuel, or spark. Same steps can be taken with plug wires but be careful and note which cylinder you are disconnecting.

 

[92BroncoS]

Well I did some more testing this morning but didn't find much. I rechecked the timing with the spout connector unplugged and it's right where I left it when I replaced the distributor months ago, dead on at 10 BTDC. I also hooked up a vacuum gauge to the vacuum tree on the intake. The gauge seemed to hold fairly steady around 18 in-Hg and when I blip the throttle it shoots down to around 1 in-Hg and then back up to around 20 in-Hg then levels off back around 18 in-Hg. From what I've read this seems like a pretty healthy reading, but heres a video of the gauge if I'm missing something.

I also verified my coil is getting 12 volts to it which it is. I was able to pull some codes from it this morning.
553 - Thermactor Air Diverter (I just removed my smog pump and lines, but also read this code shouldn't be causing me any issues)
552 - Thermactor Air Bypass (Same as above)
565 - Canister Purge Circuit Failure (This is probably because I had the vacuum line to the vapor canister purge valve to throttle body disconnected just to see if there was any change)
556 - Fuel Pump Primary Circuit Failure - (This is the only code that actually seemed like a potential cause to my problem. Seems it can be a multitude of causes, but I'm assuming a possible short or grounding issue somewhere? Sounds like most people with this code it doesn't run at all but mine does just misfires...)
558 - EGR Vacuum Solenoid Failure - (Again this is because I had disconnected vacuum to the EGR to rule out the solenoid keeping it stuck open)

The next set of codes are all transmission codes which I suppose is a separate issue, but I'm also confused by them. I have gotten a few of these in the past, but over the course of 20k miles and 1.5 years I've never had an issue with the transmission so not sure what's with these.
621 - Shift Solenoid 1
622 - Solenoid 2 Circuit Failure
631 - Overdrive Cancel Light Circuit Problem
629 - TQ Clutch Circuit Failure
626 - Transmission Coast Clutch Solenoid/Circuit Failure

@Tireguy The miss is almost non existent at idle, but I did think of unplugging the injectors or spark plugs one at a time and seeing if I can pinpoint which cylinder is having the issue which I may try next.

I'm really at a loss over this, I'll be honest diagnostics is certainly not my strong side. However I've kept the Bronco as a reliable daily for 1.5 years at this point and I plan on many more! Any other advice would be greatly appreciated, I really need to get this thing back to running order as it's my daily driver and only vehicle.

 

[leomags]

Probably not the cause but reading my posts you will find I had a bad lobe on the cam and that was causing one valve to not open or close properly. Go figure on a 1991 351 engine with 37,000 original miles and well cared for at least by me...

 

[paul rondelli]

I had a missfire in my 1996 (302 with a 5 speed) under WOT. Just some hesititation when hitting the accelerator. I thought it was spark related ... so I too replace plugs,cap,rotor, coil. Still had the hesitiation. After research i thought maybe the PIP sensor on the distributor. Since I was rocking the factory dizzy.. I swaped out the dizzy with a Auozne special. It runs great now.

The shaft of my old dizzy showed some signs of heat build up and it did not spin as freely as the new one. My root cause analysis was that the old dizzy would build up resistance and heat and ... the faster it spun.. the more heat.. causing the PIP sensor to provide bad RPM data to the ICM

 

[92BroncoS]

@leomags I’m thinking my valves are okay based off of my vacuum gauge readings but I’m also not certain since I’ve used a vacuum gauge very few times so I’m certainly no expert hah.

@paul rondelli my truck misfires under very little throttle, or at WOT also. Also I just replaced my distributor back in January as the bearings on the original were toast and it was making noise. I think I still have the original somewhere I may swap it in just to rule out the new one having any issues.

 

[miesk5]

Yo 92,
J1 DIAGNOSTIC TROUBLE CODE (DTC) 556:
CHECK FOR VPWR TO FUEL PUMP RELAY
DTC 556 indicates a fuel pump primary circuit failure.
Possible causes:
◾Inertia Fuel Shutoff (IFS) switch not reset or electrically open (if in primary circuit).
◾Open or shorted circuit.
◾Damaged fuel pump relay. (inspect it or swap Horn Relay into socket - assuming horn works)
◾Damaged Powertrain Control Module (PCM).
•Disconnect fuel pump relay.
•Key on, engine off.
•Measure voltage between VPWR circuit at the fuel pump relay vehicle harness connector and chassis ground.

EEC IV 60 PIN CONNECTOR Pins & LEGEND @ Ford Fuel Injection by absent member Fireguy50 (Ryan M)

 

[miesk5]

Yo
Fuel Pump Relay Wiring Diagram in 92-95 Bronco & F Series @ 1983 Ford Bronco Diagrams picture | SuperMotors.net