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Some Performance Issues and Other Questions


Thrashingrage

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Hey folks,
Just cruised around the Corral and Ranger Station some and saw some of the info under the 2.9 Engine. I figure now is as good a time as any to ask some questions about some 'problems' that I've been having as well as ask for advice on a few other things.

First of all, I have a 1990 Bronco II with the 2.9 engine, Mitsubishi FM 146 manual transmission, and stock axles (guessing 7.5" rear and Dana 28 front). Just about every other major system, excluding the air intake (more on that in a minute) is stock in this truck. I bought this vehicle from my best friend's cousin back in June. As far as I know, it could have anywhere from 175,000 to 300,000 miles on it. It starts every time and runs like a champ. Before I bought it, me and a friend checked the compression and all of the cylinders tested somewhere in the 150 PSI range. I have spent the past two months thoroughly cleaning the interior, performing long-neglected routine maintenance, and otherwise undoing the half-assery that the previous owners foisted off on this Little Bronco. Speaking of them, the first (as far as I know - I'm at least the 3rd) guy supposedly had the transmission rebuilt around a decade ago, for what it's worth. Anyhow, I've replaced the brake master cylinder, PCV valve, valve cover gaskets, ignition wires and spark plugs (the old ones were about a year or two old and all had nice even, clean burns on them), and a whole slew of other little things. I have drained and refilled the rear differential and transfer case, as well as used a suction gun to get all the old gear oil and dirt out of the front differential. I'm planning to drain and refill the transmission as soon as I can get a new seal for the oil pan. I had the local Ford dealer re-do the alignment and put some camber bushings on, as well as adjust the rear brakes. The A/C was also recently converted over to R-134. I also had a local gearhead put new pads, rotors, and calipers on the front brakes, as well as lube the spindles and repack the wheel bearings. Just yesterday, me and my dad rotated the tires and repacked the lock-out hubs (they are stock, which some have told me is rare for the Bronco II).

Anyhow, I have several concerns. The most significant is that my acceleration, particularly at high speeds, seems to be minimal. Getting up to 65 or even 55 when pulling out on the highway is slow at best (Granted, it is entirely possible that this has to do more with the gear ratio than performance issues. I have only owned the truck for a few months and just may not be used to it yet, especially since I did not previously have a truck or SUV as a daily driver). To boot, I have been getting what I think are small backfires at high speeds. These seem to happen only when I either accelerate hard or suddenly decelerate, and do not happen very frequently. They may not be backfires, but the noise that I hear sounds about equivalent to hitting a steel gas tank with a rubber mallet. It is highly likely that they have something to do with the truck's air intake. Back to the previous owner. He installed a Weapon-R cold air intake on the truck and jury-rigged the thing to the max. It clamps on the throttle body from a cut piece of the stock air-cleaner-to-throttle-body hose and comes out about right next to the relay box on the passenger side. Now, this would be fine, except for two things. First of all, he cut off the exhaust pipe right behind the catalytic converter. Though it may have made the truck sound 'cool', it sure as hell didn't help it run cool. To that end, I had a local muffler shop put a Flowmaster on and run a single 2 1/2" pipe out of the back. The second, and potentially bigger issue, is that the guy left the bottom part of the stock air cleaner attached, along with the vacuum hoses to/from it and the intake (or maybe it's an outlet, I'm honestly not sure) to/from the exhaust manifold. In other words, the bottom half of the air cleaner is in there along with with the gaping port that runs to the RH exhaust manifold and connects to the radiator core support. Based on my as-of-now-incomplete understanding, this apparatus may have something to do with the thermactor. If so, I am wondering if it is possible/likely that the backfires are resulting from a lack of air entering the exhaust manifold or possibly a bad thermactor bypass valve or check valve. Now, I also read something in the Library about installing a K&N filter directly to the throttle body. According to that, you are supposed to disconnect the vacuum hose from the exhaust manifold hot air hose and plug the vacuum hose at the engine. As far as I can tell, that vacuum hose runs directly from the baffle/core support intake and attaches to a port on the Weapon-R intake. thus, disconnecting it may not do anything except necessitate another plug for the intake. On that note, I have ordered a K&N Apollo intake to replace the jury-rigged setup that is in there and at the same time rout the intake to a better location, hopefully by the radiator so it can get some more air. When/if I do this, I will need to find a way to fix the issue with the exhaust manifold air hose, etc. and hopefully solve my backfire issue. The other possibility on this one is a vacuum leak. The jury-rigged intake may have one somewhere, especially considering that the vacuum hose to the exhaust manifold isn't plugged. I plan to check for vacuum leaks today for good measure. Any input from you guys with more experience is greatly appreciated on this one. A final note on this: it is entirely possible that the slow acceleration has to do more with the gear ratio than performance issues. I have only owned the truck for a few months and just may not be used to it yet, especially since I did not previously have a truck or SUV as a daily driver. However, it seems more likely that it is a performance issue, especially coupled with the backfires.

You guys are probably rolling your eyes by now, but my next question has to do with timing. Just the other day, I checked the timing in hopes that it could be the cause of my backfires and acceleration issue. I disconnected the spout connector and hooked the light up to the #1 plug and it read exactly 10 degrees below TDC. Based on my situation, I am wondering if I would benefit from the timing change listed in the Library, which is 12 degrees below TDC if I'm not mistaken. If I were to advance the timing by two degrees and run premium, would I risk destroying my stock valves? I am hoping that if I did do this it would either compensate for my current lack of acceleration if not add some performance once I get the other issues resolved.
Finally, it is worth noting that I have to make a 700-mile round-trip to New Mexico in about 20 days. I would truly like to take the Little Bronco for a long spin and am hoping to get at least a few of these problems, especially the ones that are going to trim my mileage and acceleration, licked before then. again, a hearty thanks in advance to any of you who have read this entire post, much less feel like tackling some of these things.
Thanks again and happy driving,
Josh
 


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adsm08

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Welcome to the forum man.

I really doubt your "backfire" issue is related to the intake setup since mine is very similar in it's missing/modified parts, and I don't have this noise issue. I'd get an oxygen sensor in there first.

A timing bump will help put some pep back in her too. I ran 12* on my last engine which was very tired and it helped a bit. I have a fresh engine with a hot cam in there now and run it at about 15* base advance with no ping. I absolutely would NOT run premium in a 2.9L. Premium has no benefits for this engine and if you are already having performance issues, or at least perceived ones, putting a fuel that requires more compression to burn properly and has a higher resistance to burning in the tank will not help that at all. The only reason to run anything but 87 in a 2.9 is if you are already having ping issues. It won't destroy the valves, but it will probably damage the cats.

Also, gears are crucial on these things. 3.45s are awful, 3.73s are ok, 4.10s are golden.


Finally, you don't really need a gasket for the belly pan on the trans. Just put some good RTV on it and clean the surfaces well then let it sit for an hour or so before refilling.
 

Thrashingrage

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Thanks for the welcome and the help man. I will test that O2 sensor tomorrow and see what the deal is there. Thanks for the tip on fuel, I'll try the timing bump and some mid-grade and see what happens. Just to clarify, are you saying that using premium will risk cat damage, or did you mean that mid-grade will? Anyway, thanks again for your input. Any other advice that you or anyone else has to give is genuinely appreciated.

Josh
 

PetesPonies

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Octane is just the fuels resistance to burning. The higher the octane, more resistance. That is so hot spots or high pressure doesn't light the fuel, instead of the spark. It has nothing to do with power. If you are buying higher octane, you are just throwing money out the window. It's doing nothing for you. Check your timing and replace the O2 sensor. Of all the sensors, the O2 is one that degrades. They should be replaced every 50K miles to have them operating well.
 

Thrashingrage

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Thanks for the help guys. Adjusted the timing to 12 advance the other day and it seems to have helped. I'm going to put a new O2 sensor in tomorrow and hopefully will have a non-jury-rigged air intake setup in the next few days.
While I'm thinking of it, what are people's opinions of synthetic fluids around here? Considering that this truck has some miles on it, will using them benefit performance or just cause premature wear? I'm not exactly hot to covert all my systems at once, but I'm thinking of putting synthetic gear oil in my transmission since I have to drain it anyway. Anywho, thanks again for the help. Really appreciate everyone's input and advice.

Josh
 

country0001

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I'm a big fan of running what the manual says. Use Motorcraft oil filters, a quality SAE rated oil, and keep your air filter replaced.
 

Thrashingrage

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Thanks for all the help guys. Replaced the O2 sensor, set timing 12* below TDC, and put a K&N intake on there. The Little Bronco seems to be running a bit better and have better acceleration response. I plan to run a snorkel from the intake to somewhere by the fan so as to turn it from a 'hot' air intake to a 'cold' one. Thanks again for the help, and, as always, any other tips are more than appreciated. Still trying to figure out what to do about synthetic fluids, but that'll have to wait for a while, as I have some standard gear lube that needs to get used first. And Country0001, I definitely agree on the routine maintenance point. I have been pretty religious about following manual recommendations - I've done everything from draining the transfer case to lubing weatherstripping.

Thanks again,
Josh
 

clvanhorn

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find yourself a stock intake box and tube... it goes up and behind the grille... it is already cold air from the factory, and is the best performance you're going to get as far as intakes go on those engines...
 


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