HungryBearCub

First, I have posted before on a different thread but it is several weeks old so I am posting my new problem as a different thread.

Ok, I know I had to have caused the 4 new problems. I took off the dash not long ago to replace the heater core. Now I have heat. I also have a hissing noise behind the dash, my check engine light came on, low idle causing it to shudder at about 500 - 600 rpm, and last night I developed a harsh shudder that attacks whilst hitting the gas (mostly hitting around 40 or 50mph and up).
The codes on the check engine are: P0152, P0156, and P1131
I have a feeling that the codes, hissing, and low/ rough idle are related and that the hissing is a vacuum leak.

Now to what I do not know. Number 1, how do I detect/repair a vacuum leak and any suggestions as to where to even start looking under the dash. The sound seems to be coming from around the fire wall. Cant tell if it is high or low. Sometimes it changes in intensity when I punch the gas (it gets lower like less pressure when i hit the gas and i want to say it is at about 40-50mph and up where it shudders now).
Number 2, Could the shuddering while accelerating be a new symptom of the vacuum leak? It has been over a week since the vacuum leak started.

I guess I need to tell ya a lil bout the truck. It is a 98 F150 Lariat V8 5.4L 4x4

If there is anything else you need just ask.

bones2o21

Vacume leaks can very tricky too find but easy too repair. also a vacume leak can do what u stated above and alot more strange things. the best way iv found too trace a vacume leak is get a can of carb cleaner and spray around the area u hear the leak with the engine running u should here the idle of the engine change if theres a leak in that area. now thats good if ur hearing the sound in the engine bay i wouldnt go spraying up the inside of ur cab with carb cleaner LOL. for that i would just see if u could get ur hand up under the dash and get a hold of the vacume lines andfeel emand move em round u should hear the hissing sound change as u move the lines around also u might just pull the heat/cool cluster from the dash and see if u can find anything that way.

Also as loud as u make it sound the vacume line is either disconected or broken completly so should be pretty easy too find visauly. i say should but i know how tight some areas can be on that truck. if its a line broken then i just find a rubber hose just big enough too fit over the vacume line while making a good seal on the line. thats only if u cant just replace the whole line. if u can get the whole line off then just take it up too ur nearest autozone and they should be able too get u a new one too replace it.

HungryBearCub

is there a diagram of the vacuum tubes under the dash on the cab side of the firewall perhaps? I am going to have to take the dash back off to inspect those areas on Thurs. and it would be a great help to have some form of reference as to the locations I would be needing to inspect.

cvz_finezt

u have to do a smoke test to detect a vacuum leak. it will make it a lot easier to fine doning this.

BigIrish

If you suspect the leak is inside the cab for the HVAC system, then you should be able to see which vacuum lines run to the firewall. Disconnect and cap them, then start the engine to see if it improves.

As far as I know, vacuum powers the mode selector baffles (floor, panel, defrost, etc.) and possibly the temp. blend door.

HungryBearCub

Last night I was looking online and someone had 2 of the same codes and mentioned it had to be a leak past the MAF so I went outside and checked the MAF, it was good so I cranked the engine and started just jiggling hoses and hoping. I found a line that was disconnected at the throttle body. I must have pulled it loose when I took the hoses off of the heater core. So glad that problem is fixed. Thank you all for the help!!

BigIrish

Glad you fixed it. Vacuum leaks cause huge problems for newer trucks with fuel injection and vacuum operated accessories (axle / hub lock-outs, HVAC doors, emissions equipment, etc.). Basically, the MAF and 02 sensors send conflicting info and the computer cannot compensate for the un-metered air coming in post MAF so the engine gets buggered and doesn't run for crap.

Chacka

Mine was disconnected too. Mine still has a small shudder at 50-60 So Im still not out of the woods with mine yet. I have checked and replaced any faulty lines, thus far.

HungryBearCub

When you figure out what happend to yours causing the shudder give me a holler cuz I noticed today it still has that same shudder. The original problem is gone but it seems the second it drops into 3rd gear? it starts to shudder. I am waiting the check engine light to come on to give me the next clue. I suspect it is a coil pack gone bad but I really haven't a clue. Still low idle but doesn't seem to shudder at the low idle.

BigIrish

Specific speed shudder is out of balance tire, warped brake rotor, or u-joint.