Just wanted to post eveything I know about what is happening in hope that someone will be able to tell me what to replace first.

The truck has a steady idle and does not idle rough in park, but when I put it in drive and have my foot on the brake is when the truck starts to get a shake to it (rough idle). However the idle is still constant, it does not change in the RPM's. What should I replace first. Thanks to all who can help.

 

Don't bank on this, kinda a long shot. If you push the brake in neutral and it does NOT shake, this is NOT the problem. If regardless of gear, the brake application causes the problem, it could be it. If it happens cold and not after being warmed up, it's likely.

Ruptured brake booster diaphram. They can cause a vacuum leak when pushed. If cold, before the O2 sensors warm up, the fuel being controlled by the MAF inputs and this leaked air not being metered, would cause the truck to run lean therefore rough. After it warms up, the O2 sensors wouldn't care if the air came past the MAF or not and it wouldn't run rough when hot with the brake depressed. In either case (cold or hot) the idle speed would be corrected by the IAC.

If it only happens in drive with the brake on, there may be vacuum hoses or wires that are stretched when the motor leans into the motor mounts, especially if the truck is old and the mounts are getting soft. The computer does sense the brake being applied, for decelleration enleanment mostly, but I can't think of any changes the computer would make for the brake being applied stopped at idle.

 

WOW. It only does it at a complete stop with my foot on the break and it has to be in drive. Does not matter if it has just been turned on or if I have had it running for 30 min. I just recently had a body lift taken off of it and thats when it started.

 

OK, it was a long shot. At least I had a way to tell that wouldn't be it. Yeah, I'd think first about what moves when the engine's in gear with the brake on before I started thinking about engine strategy. If you have someone (who actually likes you and won't run over you) hold the brake and put it in gear, several seconds later in neutral, etc. while you watch the engine move you may see it doesn't move enough to matter or you may see what's being crushed, pulled, etc. Especially after a lift being removed.

 

i have the exact same issue... a slight shudder at idle... here is what you can do to eliminate the brake booster..... try and put it in drive, and put the parking brake on as far down as you can go to assure it wont go anywhere on you... still doing it?

is the shaking very slight? if so, and it only does it in gear... i have been told by a million people that it is normal in these trucks... and to not worry about it... i know people who spent a LOT of money at the dealer trying to figure out what is wrong with it, and to no avail.. they just tell em thats how it is... so if its a small and barely noticable (as in, if you turn the radio on a little bit you dont even notice it) just leave it be... IF it affects your gas mileage, and/or you are losing power or stabil performance, then have it checked out...

 

Bonefish,

I have the same exact problem with my 2001 Expy 4.6. Only 47k on it. Idles ok in Park but is rough when it's in drive. The RPM's stay consistent; no fluctuation. It accelerates fine with no loss of power. I think I've decided to just live with it. if you figure anything out, please let us know. I did notice that you live in Florida as do I. Maybe it's the heat down here!

Anthony

 

I think I am going to try and replace the EGR valve and the IAC valve.

 

This is what I have so far as possible remedies to a rough idle:

1. IAC valve
2. EGR valve
3. TPS Sensor
4. Leaks in the vaccum hose

so one also said check the coil packs!! What are those, where are they located, what do they do, and how do I check them?


Thanks

 

You really should take it to a reputable repair shop and have them look at it. I have seen people spend a lot of money guessing and then end up bringing it to me only to find the fix was much less than they had already spent guessing.

 

I agree with toptech. In just a few weeks of hanging out here, I have seen several examples of people who have done their own diagnosis, some with help from here, and saved some money. Gold old American enginuity. But I think I've seen many more cases of people spending more money, not to mention time and frustration, on problems they can't diagnose than taking it to a dealer. And the people here are a lot more mechanically inclined than most automotive forums.

In general, people here seem to think that a code from a code reader can be used to derive what part needs replaced. I can say with some certainty that this is NEVER true. Even things like "Throttle Position Sensor B Circuit Input Low" are more often wiring than a bad TPS. Certainly a misfire or lean condition is just a direction to start diagnosis from, not a part to replace. Traditional diagnosis skills are just as important as ever, the computer hasn't even started to replace it, just save a little time with initial direction.

I don't work for a dealer but this is still just an opinion. Dealers have several advantages over independent shops. They see the same vehicles day after day instead of a little of everything so they develop more product knowlege and recognize pattern failures far better. They are usually have more recurrent training and again it is more focused on just one line of vehicles. Having their own parts department, hotlines to the OEM techs, OEM required specific tools, etc. attract techs so the dealers usually have first crack at the best techs. Dealers shop rates are high but they may make up for it in less time (not all diag time comes from the flat rate for a repair) and doing the right thing more often than less expensive shops. Finally, ocassionally the dealer will repair something under a campaigned TSB or extended warranty that the independent shop will charge you for.

If you are lucky enough to have a competent inexpensive independent mechanic or shop in your area, excellent. You are far luckier than most.

Finally, there is quite a bit of information that hasn't been available to the independent shops. The new CARB rules that require manufacturer's to make that information available to indy's will be federal law soon. That will help. But so many times, the indy shops don't have the ability to reflash computers, the OEM scan tools to perform certain tests, or required specialty tools that they have to refer the customer to the dealer anyway.

 

I do not have a problem with that, except I took it to a Ford dealership and had it hooked up to the machine for $100 and they told me there was nonthing wrong. Should I try another shop?

 

OK, I take it back. They have some idiots there too.

 

YAW is right on both his replies.
If what you say is what the dealer done, how the hell can they see the problem by just looking for the codes and not reproducing the problem as you discribe it it happens for you!
The fault may not even set a code.
Time to use heads for more than a place to put a hat.

 

I agree. I think I will put one of the mechanics in the truck with me this time.