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Discussion Starter #1
I had a new 351w put into my 1989 bronco, and I am now getting 6-7 mpg city and highway. The idle spikes on startup (even when warm) to about 2000- 3000 rpm for 5-10 seconds, then goes back to normal idle.

The only thing I have noticed is a broken vacuum line (the pink one) to my Air bypass valve. Could this be causing these problems? My throttle also sticks, it doesn't go all the way down unless I "snap" the throttle (quick press, then snap my foot off of it)

I'm getting less mileage then I did with my old engine with bad head gaskets!

Help! This is my daily driver, and the fuel costs are tremendous!
 

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Any vacuum leak can and will cause idle issues as well as a myriad of other things. If you find one broken vacuum line there are probably more. It also sounds like the throttle cable needs lubed or replaced.
 

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Discussion Starter #3
Replaced the throttle cable, but it still happens (I thought it may have been a worn out spring). It happens when the throttle cable is disconnected, too, and I manually operate the throttle. Is it possible to have a sticky throttle plate?
 

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Replaced the throttle cable, but it still happens (I thought it may have been a worn out spring). It happens when the throttle cable is disconnected, too, and I manually operate the throttle. Is it possible to have a sticky throttle plate?

Yes it is. You can take it off then try to clean it. Otherwise you may need a replacement.
 

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This is an EFI motor or not , just wasnt mentioned in the description. Think they stop using carbs in 1987. Silly question I have did you reset the KAM in the PCM. Disconeccting the batt cables does not always reset the KAM.

I agree on throttle plate too if its sticking need to try to clean or replace it. Seeing that its going on 20 years old be safe bet to go ahead and replace it. Has 20 years of wear on it more than likely the shafts are worn out going leak air around them too. I would also look for any vaccum leaks using carb or brake cleaner with the engine running. Spray carb or brake clean around the intake with it runnning see if the rpm and way the engine is running changes. IF it changes theres a vaccum leak in that area. Did you set the timing with it running with spout removed. Not mistaken during these years try set the timing without disconnecting the spout cannot get it set properly.

Last and craziest thing I seen it happen too if has catilytic convertors are you sure they arent clogged or partial blocked off. Seen it before with blown headgaskets on gas engine from all the contamination in cats cause them to melt down and clog. Same for the o2 sensors might not throw a code but they can be contaminated.
 

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2-3000 rpm on startup for 5-10 seconds, I do believe, is normal. Mine does it always and I get 14-15 mpg mostly. I can't help thinking the timing is way off.

This might be the wrong way, but I set the timing WITHOUT the engine running. Engine stopped, remove spout, adjust distributor, re-install spout. Start engine and listen for ping while driving. Trial and error I know, but I don't want to snag a sleeve or drop a wrench in that spinning serpentine.
 

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Also a fresh engine will typically get terrible economy for at least the first couple tanks of gas as everything runs in. Not saying you might not have an issue, but if you're reporting 6-7 over one tank, have patience.
 

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Replaced the throttle cable, but it still happens (I thought it may have been a worn out spring). It happens when the throttle cable is disconnected, too, and I manually operate the throttle. Is it possible to have a sticky throttle plate?
You damned straight it's possible Broncoman79

Like rla2005 mentions, fix the pink line and get a mechanic's stethoscope under there to track down anymore vacuum leaks.

Right on the throttle body there is a return spring wound round the TB , which often breaks and this causes the higher or sticking idle.

Either wind it round again and bend it to hold, or get into a yard and get another used one, paying attention to the spring.

BTW, at 6 mpg you can buy a new throttle cable every day and still save money.

Before you get into the Sixlitre mileage modification and bigger Y-pipe etc, fix what's broke first.

How old are your plug wires ?????? Old plug wires are a HUGE mileage killer. Worn cap and rotor. Whose plugs are your running ?

Sixlitre
 

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Discussion Starter #9
Replaced the throttle body with one from the junkyard (it was in really good shape), and it still doesn't want to idle down all the way when stopped. I have to "snap" the pedal to get it to close the throttle plates. Why would this happen with 2 different throttle bodies? Could it be my cruise control cable?

Another question: The throttle body I picked up has a small hole in each throttle plate (it was off a 1991). Would this cause any problems? The only thing I have noticed is less deceleration when the pedal is released.
 

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Replaced the throttle body with one from the junkyard (it was in really good shape), and it still doesn't want to idle down all the way when stopped. I have to "snap" the pedal to get it to close the throttle plates. Why would this happen with 2 different throttle bodies? Could it be my cruise control cable?

Another question: The throttle body I picked up has a small hole in each throttle plate (it was off a 1991). Would this cause any problems? The only thing I have noticed is less deceleration when the pedal is released.
Disconnect the cruise cable and try it Broncoman

Check out this thread on the high idle cure. Halfway down the page I respond to a guy who describes the plate that fixes it;

http://fullsizebronco.com/forum/showthread.php?t=63592&highlight=Miesk5+sixlitre+IAC&page=77

Al (Miesk5) goes on to list it's uses from Ford

Sixlitre
 

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Check the IACV (Idle Air Control Valve)?

Ya more than likely have a pretty substantial vacuum leak somewhere, as already said above. Go with a stethoscope or some type of proper aerosol and start chasing the leak. Did you drop the engine in yourself or did a shop do it?
 

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Check the IACV (Idle Air Control Valve)?

Ya more than likely have a pretty substantial vacuum leak somewhere, as already said above. Go with a stethoscope or some type of proper aerosol and start chasing the leak. Did you drop the engine in yourself or did a shop do it?
You're right a vacuum leak will kill his mileage in short order (performance too).

That IAC spacer plate(from Ford, as a GM like engineering band-aid) has a bleed off to fine tune the high idle problem (that so many of these engines had) and get the idle down on "good running" (no vacuum leaks) trucks.

I kind of assumed our man had gone through the essential checks, maybe he hasn't, as you suggest.

Lets hope, together, we can get him better mileage than his current 6 mpgs:shocked

Sixlitre
 

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Discussion Starter #13
Ok, replaced EGR and tube, and fixed all vacuum leaks. I'll be doing a mileage check soon. However, now I am throwing a code 34. "EVP voltage above normal closed" or something like that. Isn't the EVP the thing that sits on top of the EGR? I replaced that a year ago.... Or is this where the color issue comes in?
 

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Ok, replaced EGR and tube, and fixed all vacuum leaks. I'll be doing a mileage check soon. However, now I am throwing a code 34. "EVP voltage above normal closed" or something like that. Isn't the EVP the thing that sits on top of the EGR? I replaced that a year ago.... Or is this where the color issue comes in?
Disconnect the battery for half an hour, have a beer and dinner and then reconnect and try the codes again.

Sixlitre
 

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have you checked the amount of vacuum you have going to your map.? cause it controls alot of the engine managment. make sure it goes strait from the map to the upper intake.
 
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