So, my 1995 5.8L Eddie Bauer Bronco with ~130K miles stumbles/misses/bucks under light acceleration or under load or climbing.

My old 1993 5.8L Eddie Bauer Bronco did that too, but I don't remember how I fixed it or even if I did fix it before I sold it.

I thought I found the answer using the search feature here and on youtube last night - a dirty MAF sensor - but the 1995's don't have a MAF sensor. The 1995's only have a MAP sensor which is on the firewall. And apparently that is a misnomer because it isn't in the "manifold" so it isn't measuring manifold pressure at all but is really a bariatric sensor measuring ambient air pressure.

I did notice that the EGR valve seems to have been replaced by previous owner (anyway it looks newer than some of the other components) and the visible vacuum hoses look good (I haven't exhaustively examined them yet, however, and certainly not with carb cleaner or smoke or whatever).

And yes, no codes. And in that latter regard, I am assuming that the 1995 is OBDI but how can I tell for sure? Also I am assuming that my prior 1993 was EEC IV, so the same exact symptom on both could be caused by completely different things?

Also, the spark plugs, wires, rotor and cap could all be original for all I know. However, I know this, on the 1993 to cure the same exact symptoms I had the stealership replace the fuel filter in the fuel tank and on the frame rail, do a complete tune up with new plugs, wires, rotor and cap and ... it didn't fix it.

So I don't want to just throw money at this problem. And any logical/stepwise approach to troubleshooting this symptom would be greatly appreciated.

 

I'm a bit confused... is there any driving mode that it DOESN'T buck?

No. The MAP sensor has a hose to it from manifold vacuum... if yours doesn't, you found a problem!

49-State 5.8L's were OBD-I, but CA may have been OBDII for 1995. Look on the driver's side fender and see if the odd-shaped OBD-1 (EEC-IV) test connector is there... Also, I assume a OBDII would have the data link connector under the dash above the brake pedal area?
It's also possible that the certification label on the door frame may say OBDII, if it is.
Also, an OBDII would have more O2 sensors than an OBD-I, right? The OBDII would have sensors after each catalytic converter, besides the one(s) before them. IIRC, my 1994 has one O2 sensor total.

 

It's a 49-state truck from TX with a single O2 sensor.

 

Then it's OBD-I, and that MAP sensor should be hooked up solidly to manifold vacuum

 

Unless the truck is MAF in which case there is no MAP sensor.

 

There is no MAF. There is a MAP. Despite what I said incorrectly earlier, the MAP sensor has a hard line to the dealy which is connnected to some kind of electrical hub which then must connect to the manifold. (IIRC.) I am going to post a picture of the engine compartment.

And hopefully if I can get the tile done in the master bath shower tomorrow, tomorrow night I look forward to Conaski pyrotechnics with the hood light bulb removed with my wife in the driver seat with the emergency brake on and her foot on the brake peddle giving it gas and me with video camera on hand on the side looking for 4th of July spark plug wire fireworks.

 

This sounds exactly like what my 91 is currently doing. You described the symptoms alot better than I did. Have you been able to narrow it down to anything?

 

sgted1974: Ok, I cheated. Because it is a Bronco, I posted it here. But because the 5.8L is used in the F Series also, I posted it over there too. So take a peek:

https://www.ford-trucks.com/forums/1...-climbing.html

Don't tell the moderators. They don't like duplicate threads. And do what Conanski says. He has helped me before and knows his stuff. When I get done with this stupid home improvement tile job, I will be following Conanski's recommendations and will update with my results. Good Luck!

 

replacing the cap/rotor, plug wires and spark plugs although a pain at times is something anyone with mild mechanical ability can and should do to save $$$ as the dealership will typically charge mucho$$

Also check the timing, make sure the engine is up to temp then remove the spout connector and hook up the timing light accordingly. you should be at 10* btdc, it may be necessary to rub a white or yellow crayon onto the timing marks on the balancer so you can see it easier.

double check all vacuum lines and also consider that the Cat Converter if original may need replaced as most only last 80-100k miles to begin with.

checking fuel pressure at the rail and replacing the fuel filter that is located on the frame below the driver side.

 

Thanks Kemicalburns! I replaced the catalytic converter this year with a stainless Y-Pipe from DEC which I bought on ebay.

I cannot say enough good things about DEC. It took three shipments to me and a discussion with the assembly line shop foreman, for them to get it right (a 49-state TX truck with one O2 sensor in California (CA vehicles had two O2 sensors) which required the California specific heavy duty catalytic converter with the stainless steel CARB bar code welded to the converter).

DEC stood right by me until I got the one which fit my odd application.

I owe the forum a write up with photos when I get the chance.

 

I have had two threads going recently:

1. Engine Surge with Defrost or Mix

2. 1995 5.8L Bronco stumbles/misses/bucks when accelerating, under load or climbing

Last week before driving to work I did what Zombieslayer and Conanski advised and disconnected AC compressor (by disconnecting the two pin electrical connector at the AC compressor).

1. This totally fixed the engine surge.

2. But for some reason, possibly related, possibly unrelated the engine also did not stumble/miss/or buck on the way in and from work or during the rest of the week (all flatland driving). I accelerated harder than I normally would to get it to stumble, but it didn't.

This weekend I drove from Sacramento, CA up Hwy 50 through the mountains to South Lake Tahoe. If you aren't familiar with this route, it is a steady climb. The engine stumbled twice requiring me to manually downshift out of overdrive to 3d, and it didn't stumble in the lower gear (which was my previous experience). At other times it shifted out of OD to 3d or from 3d to 2d without stumbling first - operating at is should.

I then drove from Hwy 50 to Hwy 88 and uphill through the Carson Pass. This is a very steep uphill grade. The engine stumbled once and only once.

This is a phenomenal improvement in engine performance. Previously, the engine would stumble so often that I would have to drive a significant part of that Hwy 50 uphill route in 2d to prevent it from stumbling.

Zombieslayer said this to me in a PM: [/QUOTE] By unplugging the pigtail on the compressor you are killing the electrical signals (and the A/C while the pigtail is unplugged) so the compressor is just sitting idle. The clutch still spins which allows the belt to move. [/QUOTE]

So, the problem is solved, at least temporarily until I want to have air conditioning, but the mystery persists. Why did disconnecting the AC compressor help? And what would be a permanent fix?

In the interim, I would suggest that everyone who is experiencing a stumble/miss/bucking with a 90's era 5.8L disconnect the two pin plug at the AC compressor and see if there is any improvement. It is a simple test, and does not cost a cent.

Finally, on my way home this weekend, the belt tensioner broke (the entire pulley came off) and I had to be towed home. (It started squeaking last week and I thought I had a few days to get around to replacing it, but I was wrong.) I mention this only to be complete. I don't know whether or not the likely bad bearing in the pulley is related to this problem or not.

 

That's definitely interesting however like you said it's not a permanent fix.. The ac compressor is in and belt hooked up however I know it's not operational in mine. I bought it for an offroad family wagon but everything's there so once I tackled the shot trans I figured I'd be able to tackle all of its issues eventually- I have however almost always had the defroster on.. When in vent mode I can smell the small valve cover leak so I would just switch to defrost and have vent air blow on the windshiled. I never noticed if it stumbled when switched off but that's interesting and has me thinking now. It is currently at a shop being trouble shot now. I had a very long climb up the 138 on new years day and ended in 1st gear the last 7 miles just to keep the rpm's up and keep from stuttering. She made it though. And downhill and flat all the way home was 75-80 mph- without hesitation. Mind boggled

 

Sgted1974: Before the AC compressor was disconnected, the surging would stop with just the vent on. But the stumble was still there. The stumble didn't stop until I disconnected the AC 2-pin connector. I say try it.

 

And the verdict is... 2nd cat. Converter completely clogged. Problem duplicated whenever engine at operating temp...it's all about the back pressure. Cheap fix and back on the road this afternoon-

 

sgted1974: Good Job! FYI, I replaced the catalytic converter in my 1993 Bronco and, IIRC, it fixed this problem. I replaced the catalytic converter on my 1995 Bronco last Summer, but it did not fix the problem. Hence this post.