I've had this problem ever since the rebuild, but it only pops up when I drive my bronco for a while. Yesterday, I drove it ~60 miles, which is far more than normal. 1st trip was 18 miles and then it sat for 2 hrs, then 18 miles home and it ran great! Second trip was 7 miles into town, then it sat again for 3hrs, and 7 miles home. This trip home I began to notice the issue. I made another trip into town and it was a quick one to get a new battery for my tdi. On the way into town the problem began to get worse.

It feels as if I have a carburetor on it which begins to load up and it starts running badly. As I am driving 60-65 mph with my foot maintaining the same throttle level, I begin to lose speed and the engine starts to sputter. I can give it more throttle and it may stop losing speed, but it is still running badly. If I give it significant throttle, there is a 2-3 second hesitation before it seems to snap out of it's fit and then will run fine for a minute or two. Then it loops back into this sputtering mess.

Fuel pressure remains in the 40psi range, so I am thinking air or electronic. I checked the butterfly on the throttle body and it moves freely. What else should I be looking at?

Thanks!

Grounds, mass air flow sensor and throttle position sensor. Likely in that order. Since it started when you did the engine it's definitely possible a ground was missed. The symptoms sound more mass air flow sensor related to me though.

I was only involved in putting the rebuilt motor back in so where are grounds typically located?

Anyway to test a mass air sensor?

Engine is from a 95 f250 MAF.

ODB I or II? Fairly sure you should
Be able to get a reading via a scan tool, but I’d have to read up on it, maybe just clean it, use the special cleaner if you do.

As for grounds, there are the factory grounds, like starter & alternator from the motor to the frame. Also the body to frame ground the should be on the fire wall to frame. I’ve recently read that you shouldn’t stack a set of grounds to one post with EFI, that you should run a bus/ground bar instead.

OBD I

Cleaned up the intake yesterday. I first pulled the MAF and cleaned with the specific CRC cleaner, even tho it looks perfectly clean. I then took the IAC off and cleaned it out; also not dirty. I pulled the TPS off and there was no visible evidence of issues, but I think it should be tested somehow. Will research that.

I'll look for grounds you mentioned and go from there, but I would think if it were a ground, my issues would be permanent. Not show up after several miles??

Thanks!

O2 sensor leads get swapped?

I'd have to confirm that with Garry

Here are picture of the drivers side O2:



And passenger:


I didn't get a lot of time to look for grounds, but I only spotted one that attaches to the rear passenger side of the head and then goes into the engine harness. I have not located anything from the engine to the body or frame.

I tested the TPS based on this video: https://youtu.be/lJEGigONelc and it was fairly close to these readings, but maybe a little outside the parameters he states. But I know that sometimes when these gadgets warm up, things change.

I found another youtube that stated you can unplug the MAF while the engine is running. If it dies, the MAF is likely good. If it still runs, the computer has already compensated for a bad sensor. So when I tried that, the engine still ran, but I got the "check Engine Light". If I plugged it back in, the CEL went away.

Aside from finding or adding grounds, anything else I should chase down?

B1 means bank 1 and that should be on the passenger side. B2 is driver side.

They are correct.

I took it out wheeling this weekend. Was idling rough when I was preparing to load it on the trailer. Got to the campground and I left it running after I unloaded it while I jockey'ed the trailer into place. In the 3 or 4 minutes it took to place the trailer, the bronco had died on it's own. Started up again and never died on me the rest of the trip, but would begin to idle rough after idling for 10-15 seconds. A blurp of the throttle would straighten it out, but only temporarily. Seemed to idle with a load, like over obstacles, just fine.

So, could the IAC be the culprit? When I cleaned it, the valve seemed to move freely, so I am wondering if it has failed on the electrical side. Any way to test it?

Well, now I have a new problem...



As you can see above, this wasn't this way in Sept. so I must have moved things around and burned the sensor wiring.

So it's time to replace both sensors, but I do need some guidance on how to replace the connector on the harness...

Well That's No Bueno !
Do you think that was causing the loading up issue?

I would just hit up the JY that's a pocket part, its part of the 3$ gate fee

No, I was having issues before this, but perhaps I have had a bad o2 sensor. Although, I have no CEL.

Rockauto actually sells a pigtail for fairly cheap, which if that is where I order the o2 from would save me the hassle of getting to the PNP.

Either way, whether I like it or not, I will be throwing money at it.

Quick question... Just realized my old o2 sensors are 3-wire even tho the engine harness has 4. The new o2 sensors are 4-wire. Can I run these 4-wire sensors? What is the difference? Heated or not?

Engine is from '95, but even if I go back to '90 Rock Auto still shows 4-wire o2's

EFI guy came thru for me. Said I can run the 4-wire sensors. That's great news so I can begin getting this thing to run well again!!

Made my first 12 mile run into town to fill up and I am apprehensively relieved to report that she ran great the whole trip. I'm not 100% convinced...but at least there is hope that this has been an 02 issue all along. Why no CEL has me baffled...

Yes I was, but I had never replaced those since I bought the bronco 12+years ago. That is why I am confused they didnt throw a code.

It's still running great! Had ZERO issues on the last trail ride with another to come this weekend. I also plugged the EGR port on the lower head near firewall, and ran my PCV right into the upper intake. Hope that helps things a bit too and perhaps some fuel mileage will return!

JD, the O2 sensors that were in there were the three wire matches for the Mustang EFI harness, vs. the four wire that would be stock on a '95 F250/Bronco. The trucks generally ran a 2 into 1 exhaust and had the Ow2 sensor in a short cross-over pipe connecting the two halves. Since it was as far away as that, it got the dedicated ground wire for the heater.
HTH, Joe