1994 mustang 3.8 rough idle

nvroom89 · Mar 16, 2017

Hey guys, sorry for beating a dead horse with this thread but i've searched and cannot seem to find any solutions to my issue.

I just recently picked up a 1994 ford mustang 3.8 from a family friend. A little background on the car:
It has been sitting for two years and was parked because he had an issue with it not starting.

I found out the starting issue was a simple fix: bad battery cables. Replaced battery cables and did some maintenance, spark plugs, wires, coil, fuel filter, air filter, and oil change.
The car still seems to idle very rough and car shakes at idle. The only codes I was getting was p0300 - Random Misfire Detected.

I replaced the IAC (figured it was bad and had been sitting so why not).

At this point, I am scratching my head and looking for any vaccum leaks. I did find that the vacuum hose was disconnected at the selection T for the interior vents. I fixed this vaccum leak and retested but I still had an issue with rough idle.

At this point I had been driving the vehicle because my other vehicle shit the bed. I decided to bring the vehicle to a local Ford dealership because I knew a few techs there. they scanned it and found codes:
P0305 - Cylinder 5 misfire
P0306 - Cylinder 6 misfire
P0174 - Lean code cylinder bank 2
P0171 - Lean code cylinder bank 1
P1151 - Lack of upstream heated oxygen sensor indicates lean bank 2

The dealership removed the intake plenum and found the cylinder 6 injector wiring to be damaged. They repaired the damaged wire and removed and cleaned all the injectors. After getting vehicle back from the dealer the car ran worse. I found out the injectors were leaking and decided to replace the injectors thinking that they were likely part of the original issue.
I replaced the injectors and reassembled and found the vehicle runs way better while driving and on the highway; however the vehicle still seems to have a rough and shakey idle and the CEL comes back on for p0300, the random misfire detected code, two trips after replacing the injectors.

I checked for any fuel leaks and did not find any. At this point I've put a large amount of money into the vehicle and am just trying to get it to run right, or at least idle better. Any input would be greatly apreciated and sorry for the insanely long write up just figured I would give all data on parts and tests that were performed up until now.

Thanks in advance!

07GtS197 · Mar 17, 2017

First off welcome to the forums!

Now there are probably a few problems at play here. Do you have a vacuum pump? Since it runs rough at idle but fine under load its most likely a vacuum leak. If you do have a vacuum pump you could check the egr valve for functionality. Its possible that it needs to be cleaned or is damaged and stuck open. You can also pull the vacuum line off the egr while idling and feel for vacuum.

The p0171 and p0174 are lean codes for bank 1 and 2 so it would seem as though you do have a vacuum leak but the p1151 indicates a fault in the o2 system. Given these codes and the fact that o2 sensors usually throw lean codes when they fail it would seem that both front o2 sensors need to be changed. You should have rear o2 sensors too if Im not mistaken, you should replace those too.

I would still perform an egr function test first but o2 sensors will need to be replaced soon.

g36 monkey · Mar 17, 2017

I know you mentioned hunting vacuum leaks. One of the best ways to do so is to take a cigar, smoke it, and exhale into the vacuum line on the brake booster and watch the motor. Leaks will be readily visible and can be addressed from there.

It may also be O2 Sensor related. How many miles are on the car? It is possible it is starting to have head gasket issues, and this might be early symptoms. Fear not, head gaskets can easily be done in an afternoon on the car!

g36 monkey · Mar 17, 2017

Heads up, I edited the first post so people will be able to help a bit more.

nvroom89 · Mar 17, 2017

thanks for the edit and advice, i'm going to be doing a block test before the end of the day to hopefully rule out any head gasket issues. the body has about 149k but ive been told that the motor was replaced due to head gasket issue at some point in its life.

nvroom89 · Mar 17, 2017

unfortunately I do not have a vacuum pump, but I will try to disconnect vacuum line at the egr and see if I can feel any vacuum

98snakehorse · Mar 17, 2017

Have you replaced your upstream 02 sensors yet?

nvroom89 · Mar 17, 2017

alright so latest update here. I used a block tester this afternoon after work, no indication of combustion gases in the coolant system. scanned the car again and found that the p0300 code is gone but now there is a p0306 code. disconnected the vacuum line at the egr valve and did not "feel" anything from either the valve or the hose leading to the valve. i did find that the hose from the intake hose to the drivers side valve cover was broken and tried to do a fast repair of this by putting some rubber hose over the break.(i cant seem to find this hose available anywhere) and no at this point i have not replaced front oxygen sensors but i believe that may be next on the list.

g36 monkey · Mar 18, 2017

I would go with the Oxygen sensors. You might want to look into deleting the EGR system then tuning it out, unless you are in a state that does not allow it.

nvroom89 · Mar 22, 2017

ok guys so I had a little bit of free time today after work scanned for codes and the only real code I have in there right now is the p0306 for cylinder 6 misfire. ran the vehicle and unplugged the coil wire to see if it made a difference at idle. car still shakes which leads me to believe that cylinder 6 is basically dead in the water. are these vehicles known for having valve problems? ive ruled out everything else plugs, wires coil and injectors. im thinking that the valves may not be adjusted correctly or maybe I have a broken spring?

07GtS197 · Mar 23, 2017

Ive never heard of many problems other than head gaskets personally or cracked exhaust manifolds but Ive only seen that on the new edge 3.8s. You could pull the valve cover to see what the valve springs look like on that bank.

g36 monkey · Mar 23, 2017

Pulling the valve cover is a 30 minute deal. Worth it to get some piece of mind.

The v6 cars are only known for head gasket issues at about 160k+ and the manifold thing is from the a/c dripping. Otherwise they are totally solid.

nvroom89 · Mar 24, 2017

g36 monkey said:
Pulling the valve cover is a 30 minute deal. Worth it to get some piece of mind.

The v6 cars are only known for head gasket issues at about 160k+ and the manifold thing is from the a/c dripping. Otherwise they are totally solid.

Thanks for the info and all help on this issue I believe that maybe the tech that worked on my car did not really do a correct or proper wiring repair so hopefully I will have some time next week after work to remove the upper intake and check his work( possible with noid light hooked up and cranking engine over). While the intake is off this time I will also check valves and change valve cover gaskets as they are leaking anyways I'll update with any further info or findings after I get a chance to pull the plenum and explore

g36 monkey · Mar 26, 2017

Keep us updated!

nvroom89 · May 5, 2017

hey guys sorry for the delay in posting it's been a crazy few weeks. lost techs at my job and i've been pulling 6 days a week with late nights and haven't had much time to do some diagnosing. i was however able to talk to a ford technician about the issue and the ford tech says that he believes it to be a ignition control module. i explained to him that i believe it to be injector drivers or injector related because i am getting spark from the coil and to all plugs however it seems as though cylinder 6 injector is not firing. he then told me that the vehicle could not have injector drivers in the ecm as the vehicle was an obd1 vehicle and not obd2. does anyone know whether or not the ecm does in fact have injector drivers internally or am i destined to waste more money on unnecessary parts

07GtS197 · May 5, 2017

I dont know why having an obd1 ecu would affect injector drivers as the 302s had them and they were obd1... Regardless your v6 is actually obd2. The 3.8 got obd2 in 94 but the gt and cobra didnt until 96.

I dont have any more info to give at the moment but I dont think I would trust that ford tech.