A Stumper for Mechanics and Auto-Peeps (long post)

Canon EF 70-200mm f/4.0L USM

09/18/2006 06:13:41 PM · #1

Vehicle: 1988 Ford Bronco II 4WD 2.9L 6cyl engine manual transmission

Symptoms: backfires, runs weird, sputters, jerks, clacks

Sometimes it seems as though it's not getting enough fuel, and it feels like it bogs down and there's no power. It occasionally bogs down bad enough to shut off the engine. When it bogs down, two things seem to help: 1) give it more fuel by pressing the accelerator or 2)downshift and accelerate. Occasionally when it bogs down you can get it to "straighten out" by letting off the accelerator briefly then resume driving (without increasing acceleration). (Note: when it's not sputtering and carrying on, it has plenty of power and is a peppy little engine. There's no problem with cranking/starting the vehicle, either.)

Sometimes it seems as though it gets too much fuel. It'll lurch forward on acceleration (without changing pedal pressure), which leads to either a surge in speed or a very rough start from a stop. It backfires - erratically. Sometimes on acceleration (mostly when accelerating through a 'bog down'), sometimes while coasting, sometimes while decelerating or downshifting. The backfires are louder and more powerful when they happen while accelerating, but occur more frequently (and in faster repetition) while coasting or decelerating.

When the "Check Engine" light comes on, it tends to run properly, although not always. It does have its moments when it runs smooth as silk - no sputtering, backfiring, clacking, etc. But those are pretty rare.

It seems to sputter less if it's driven 'hard' - running the rpm's up to 2500-3000 before shifting seems to make it run easier, for a while.

We know it clacks because of lifter bleed down. But when it's running really smoothly, it doesn't clack. All of the mechanincs we've spoken to have said that the engines in these vehicles are prone to chattering and clacking and that it won't hurt a thing. We also know we have a slight oil leak, but it's a Ford, and that's pretty much a given. :)

Fixes: We've tried a number of things to get this fixed, but nothing really seems to help much.
Replaced:
the brain. no change.
oxygen sensor. no change.
Map air flow sensor. some change. a little less sputtering.
fuel pressure regulator. no change.
plugs and wires. no change.
some sort of relay in the dash (can't recall which one). no change.
ignition modulator. no change.
catalytic converters. no change.
had the injectors cleaned. helped, but did not cure the problem.

Could it be the lifter bleed down? the distributor? the distributor shaft? the fuel pump? bad injectors? something else entirely?

Our mechanic is stumped by this, and he had it for a week. My dad's mechanic is stumped by this. Everything that they've said might be a problem, we've replaced, and it has not helped with the symptoms.

Help!!

cryingdragon

Canon EOS-10D

09/18/2006 06:14:58 PM · #2

fuel filter is what it sounds like. it's a $20 fix

Sigma 17-70mm f/2.8-4.5 DC Macro for Nikon

09/18/2006 06:18:49 PM · #3

oh I forgot that! we did replace the fuel filter.

actually, there were 2 fuel filters: one on the rail close to the engine that got replaced, and one 'canister' type which didn't appear to have anything in it, so it was eliminated from the system.

those were the first things that got done to it (2 years ago), so I'd forgotten about it. sorry!
:)

mattforbes

Sigma SD14

09/18/2006 06:19:17 PM · #4

Restricted exhaust, have a muffler shop remove the exhaust and test drive it (will be loud), and see if that solves the problem. I work for Ford, and that will be $136.00 for the diagnostic fee.

Canon EF-S 17-55mm f/2.8 IS USM

09/18/2006 07:22:18 PM · #5

hmmm... that's a thought... but I guess we sorta did that. it was driven without the catalytic converters and muffler/tailpipe during the repair stage when we first bought it... could the exhaust be restricted at the manifold?

also had another thought - could it be the coil or the ignition switch?
:/

I just dunno...

ETA: fee?? fee??!! :P

Message edited by author 2006-09-18 19:22:41.

Prof_Fate

09/18/2006 07:48:43 PM · #6

What diagnostics have been run,and what are the results?
YOu get a check engine light - what code(s)?

One clue is when the check engine light is on the computer has gone into open loop mode - the O2 sensor is essentially ignored and the pre-programmed fuel map is being used - you'll run slightly rich. this is the normal mode for warming up, and sometimes other sensors are shot off/ignored in limp home mode.

Can you cause the condition or is it totally random?
car hot or cold, air temp hot or cold, damp or humid, etc?

Diags:
fuel pressure - is it where is should be, especially during driving? (if not..pump, pickup, corroded wiring, bad ground, intermittent relay)
TPS - any dead spots in it?
being a 19 year old truck, what about charcoal canister? these can fill and plug, or fill with gas and as the solenoids open for it to purge the engine gets lots of extra gas.

timing chain? a badly worn chain will allow the timing and cam to advance and retard, perhaps enough to cause issues.

I'd go thru every sensor and check it's readings, and then check to see that the harness from the sensor is good back to the computer - a bad connection or chafed wire can cause all sorts of intermittent issues.

eschelar

Canon EOS-7D

09/18/2006 08:20:29 PM · #7

I think I found the problem!

It's a Ford! :) Sorry couldn't resist...

Could be quite a few things. I always start at the electricals, but it does indeed sound fuel related. I never went that far when I was working at a shop, so I'll leave the real diag up to real mechanics.. or guys who know what they're talking about. :)

LoudDog

Canon EOS-350D Rebel XT

Canon EF 17-40mm f/4.0L USM

09/18/2006 08:39:57 PM · #8

you have a serious vacuum leak.

edit to add:
after re-reading it could be the fuel pump too.

You're either getting too much or too little fuel, or too much air.

Have you done any "performance mods" to it?

To test the fuel pump, pull the fuel line off, set it in a can, and turn the starter. You should get a pretty good flow. Too check the vacuum, while the vehicle is running spray carb cleaner anywhere you think there may be a leak. If the idle changes, there is a leak there.

Message edited by author 2006-09-18 20:45:03.

drydoc

Canon EOS-7D

Tamron SP AF 28-75mm f/2.8 XR Di for Canon

09/18/2006 09:23:21 PM · #9

Sarah,

Owned one for 16 years that I used for snow plowing! I has the same sorts of problems. . . often. The carburator adjustments need to be dead on. First of all, make sure those vacuum hoses are not deteriorated. Now it has been a long time since I fooled with it but: There are two vacum lines that come off the front of the carburator. To adjust it effectively you need to plug one with a golf tee (at least that is what I used) but I don't remeber which. Also, inside the air filter is a sensor. It may need to be replaced. Coming off the side of the filter is a rebreather hose with a filter infront oof it. Needs to be cleaned well. On the gas throtle on the carb is a very small set screw. It needs to be adjusted to give you the correct rpms at idle. From there you go to the jet adjustment. One other thing I remember is I spent a lot of time with a timing light to make sure that was dead on. And finally, if I remember correctly, there was one plug in the rear of the engine that was a b&%$ to get out. Often it was overlooked for replacement because it was so difficult to remove and replace.

As I said, it has been a while but maybe someone else can jump in and add to this. Now I wished I had saved the manuel!
And after all has been said, I am assuming it is a manuel carburator! The reason you are having so much trouble is that these carbs were to be run by the sensors to controll air gas and oil rebreathing. They weren't used for long.

09/19/2006 08:57:09 AM · #10

Update:
We replaced the coil and the throttle body position sensor last night and neither seemed to help with the problem.

Prof_Fate: I don't know what the codes were, but we have replaced everything that the mechanic suspected was causing the problem, to no avail. The condition is totally random, though it seems to run a little better (not correctly, but better) after the engine has run for a bit (10-20 minutes). Of course, if you turn it off for more than a minute or two, the problems starts back up again. Timing is good (we had it checked out, plus there's no starting/compression problems). Also, I'm not sure that the computer is causing the problems... one of the first batch of things we replaced was the brain. I'm assuming that's what you're referring to as the computer, unless there's another module of some sort that's throwing a monkey into the wrench works.

eschelar: :P Yes, it's a Ford! And it came with the factory installed oil leaks, too! :)

LoudDog: The fuel pump works. Both of them work, actually (the in-tank pump and the external pump). Don't know about a vacuum leak - will look into it.

drydoc: Sorry. Forgot to mention ours is fuel injected. :) I will, however, look into this 'forgotten' plug in the back of the engine to see if ours has it, and change it if it does. Would the fuel injected version have the sensor in the air filter? Or was it replaced by something else? (Anyone know?)

So basically, now we look into the vacuum system, a possible sensor either in the air filtration system or on the back of the engine, and the wiring harness. Plus a chance that it's an exhaust problem.

Any other ideas from the auto gurus out there?