Hey all. New here and I have a question about my 97 f-150.

I have the 4.6 and I think it has the 3.08 rear end. Tranny code is 18 if that means anything to anyone.

My question/problem is this. When I am coming to a stop and the tranny is supposed to go from 2-1 my rpms bottom out to around 200-300. Then all of the sudden they will jump back up to around 600 (where they are supposed to be) Also, when driving down the highway it has started acting almost like it is pulling down on the tranny. Kind of like downshifting and letting the RPM's act as a brake. Not bad mind you but noticeable. Tranny fluid is good. Proper levels, no burnt odor and a good color.

The truck has 134k on it and other than that runs perfectly. This has all been going on for about 3 weeks now. Any help would be greatly appreciated.

Thanks,
Chris

 

I would think that this much of it has something to do with your brake booster. But it wouldn't explain the rest of your symptoms. I would suggest that you go drive around, and see if your engine problems correlate with you pressing the brakes. If they don't correlate, then come back, and maybe somebody smarter than me can help you.

*Edit* After re-reading your post, I'm not so sure about what I said, but you can give it a try anyway.... Try posting this problem in the tranny section, and see what happens...

 

Chester,
I think you were actually right on with the brake booster thought. I went to a friend of mines after work today who was a mechanic for a long time. While I was driving I couldn't get the truck to do the rpm down thing. I told him about the brake booster like you suggested and he agreed that that could definitely make that happen. He then proceeded to take the vacuum hose off of the front of the brake booster and there was very little pressure when he did it. He said that there should have been quite a bit more. The truck was off and had been sitting for about 20 minutes. The funny thing was that when he put in on and I started up the truck the brakes seemed to work better. They were a lot firmer than before and all he did was take off the vacuum hose and put it back on. That one doesn't make sense to me at all. It also never did the rpm down thing since I drove home and also to get the rugrat. Not once.

I posted on another thread about a comment someone made about the truck feeling a bit "jumpy" around 1500-1800 rpms before today. Since he took that hose off and put it back on, no "jumpieness" (don't think that is a word)

The only other thing I had read about on here were the elbows after the PCV valve melting and leaking. We looked at the pcv hose and could not find an elbow anywhere. It was a solid hose all the way around. We never looked at the IAC with that elbow. I'll drive it again tomorrow to see if I can recreate anything and keep you posted.

Thanks for the help. It seemed to have been the best starting point and hopefully the source of the problem.

Chris

 

i believe there is a check valve at the point where the vacuum line attaches to the booster. taking the line off and putting it back on may have jarred it loose if it was stuck.

the check valve is there so air can only move one way. if it moves both ways then you end up with a vacuum leak.

 

Now that is some news I like to here. Thanks. After I drive it tomorrow I should have a better idea if I can get it to do the idle down thing. If not, I'll consider the problem solved and also if it shows it's ugly head again in the future I know what line to pull to maybe fix it again.

Thanks all for the great help.

Chris

 

I'm having a similar problem with my 2001 supercrew 5.4. Like you said, when I'm comming to a stop, like at a red light, the rpms will sometimes go way down like to 300rpm, then after a few seconds it will jump back up to around 650 where it idles at. Let me know if you find out what it is. I guess I should check my brake booster also. Thanks.

 

If it does it every time, then it's easy to check. Just take a pair of locking pliers and pinch the hose off that runs to the booster. Then drive it around and put on brakes like you normally would. Just be careful cause you won't have any power brakes....

 

Well actually it dosent do this the entire time I'm driving. Maybe 3 or 4 times on a 7 mile trip to work. Its weird, at some lights and when slowing down to make a turn it will do this drop in rpms and other times its fine. It dosent stall or anything, just drops real low in rpms then kicks right back up. The truck dosent shake or miss or anything, u can just hear it get real quite when the rpms drop and then all of a sudden it will go back to a normal idle. Could it be my IAC?

 

Maybe, but I don't see how it can run smooth at 300 rpm.....

 

Well maybe its not as low as 300, probably 400. I watch the tach dip under 500. Then back up to 650 or so where it idles.

 

Mustang,
You are describing exactly my problem. It did do it again today. Like you, it does not do it every time. It may 2-3 times in a row and then may not again for the 7 times I come to a stop. I'll try the vice grip thing tonight.

Thanks,
Chris

 

I do have a question. If in fact this is the problem, would a used booster from a salvage yard from any "newer" f-150 work? Not trying to go on the cheap but I have a buddy who's parents own a truck salvage. I can also get a set of used 4.10's that I am looking at.

Thanks,
Chris

 

I replaced my IAC valve today. I'll drive it for a few days and let you know if that fixes the problem. The IAC was only 54 bucks from the dealer, thats of course with employee discount. Otherwise its something like 80 bucks retail price. I'v heard of people cleaning it and having good results, but I just decided to go ahead and replace it. So far so good, but only time will tell.

 

Well the IAC was not it, the problem returned soon after installing the new IAC.

 

Just for kicks, did the "pliers on the brake booster" idea do anything?