Posted: 12/24/2008 6:10:01 PM EDT
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I have a 1991 Bronco, and am having a prob with it. I am going to post this in GD to get a few more responses, but I am thinking I will get a few better awnsers in here.
I am having some problems with my truck. When I start it up, it has a minor vibration (rough idle), obviously (to me) coming from the engine. When I put it in gear or am sitting at a stoplight, I am getting a pretty good amount of shaking. Once I start applying throttle, it smooths out, until I start cruising and my rpms drop to around 1000, then I get quite the vibration, feels almost as though I lost a cylinder, or am having a huge vacuum leak. And I seem to have a dead spot in my power from 1k to 2k. The dead spot gets bad enough that when I apply throttle, but not enough to cause a downshift, it produces almost violent shaking and feels like it is trying to stall. I am no good at engine diagnosis with things like this, and I would like some direction on where to go from here. I have recently replaced my driveshaft, and have had my tires re-rebalanced (long story), so I do not think it is a problem with that. Help? Thanks and Merry Christmas/ Happy Holidays ETA: Oh yeah, the smell from my exhaust smells like it is running very rich. Just some more info |
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1-Take some carb cleaner and spray it down each side fo the intake manifold, if the engine speeds up there is a vacuum leak on the intake.
2-Check your plug wires for resistance. If you have an Ohm meter, take the leads from the meter and touch each end of the plug wire, your readings should be somewhat similar. If not you may have found a bad wire. 3-Pull the plugs and check them out, you might even want to replace them. You may have a fouled one and it is hard to tell if you do. 4-With the engine running, you can pull a wire from a plug and if it runs the same you may have found a cylinder that isnt firing. If it runs worse, its obviously not the problem. Keep us up to date. |
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Quoted:
1-Take some carb cleaner and spray it down each side fo the intake manifold, if the engine speeds up there is a vacuum leak on the intake. 2-Check your plug wires for resistance. If you have an Ohm meter, take the leads from the meter and touch each end of the plug wire, your readings should be somewhat similar. If not you may have found a bad wire. 3-Pull the plugs and check them out, you might even want to replace them. You may have a fouled one and it is hard to tell if you do. 4-With the engine running, you can pull a wire from a plug and if it runs the same you may have found a cylinder that isnt firing. If it runs worse, its obviously not the problem. Keep us up to date. Dammit, someone told me to do that a while ago, but of course, I forgot, but I did buy a can of carb cleaner for some reason. |
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Be very careful with the carb cleaner thing. It works well to find vacuum leaks, but it also works pretty good at finding leaking, arcing plug wires.
The MAP sensor should be mounted on the firewall on the passenger's side of the engine compartment. It'll have a single vacuum hose going to it, and a three (I think) wire connector. |
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Quoted:
Be very careful with the carb cleaner thing. It works well to find vacuum leaks, but it also works pretty good at finding leaking, arcing plug wires. The MAP sensor should be mounted on the firewall on the passenger's side of the engine compartment. It'll have a single vacuum hose going to it, and a three (I think) wire connector. Ha! I didnt think about that, will do with care, and probably do an ohm check first |
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No Check Engine Light correct? Sure as hell sounds like a vacuum leak to me. Most commonly due to
a)vacuum hose leak (check hoses going to brake booster) b)MAP sensor... take it out and clean it... if that doesnt work you can grab one at a junkyard for next to nothing. c)gasket between intake manifold and heads Is the motor original to the truck? If it is, I'm going to assume its mass air flow, not speed density. If it was a swap, and somebody swapped an 87/88 Mustang motor/harness in there, swap out that POS speed density setup for mass air flow (if it wasn't done already). I've had LOTS of 5.0 mustangs and they often suffer from similar problems. Pulling the intake is easy enough and the gaskets are cheap. I say it's not spark related since it runs fine under acceleration/above 1k. Oh and if it smells like it's running rich and you still have catalytic converters on the truck my moneys still on vacuum leak/MAP sensor. If you don't have catalytic converters, it's always going to smell like that. Best smell in the world 
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Not sure if the truck engines have one. My guess is that the Idle air bypass valve may be stuck or clogged. If your not getting enough air into the engine at part power, or are choking on on decel you may get the vibration you talked about.
Had to replace one on my 92 mustang for a rough Idle several years ago. Just another place to check. |