94sc runs bad at idle and light throttle, running out of ideas!

birdsc

Registered User
So my 94 sc is running very bad at idle and low boost/throttle. It stumbles as if it had a bad wire or a plug.

Immediately when you give it gas and boost rises like from vac 10 to vac 5 it runs fine and from there to wot it runs fine. It doesn't seem to make a difference if the engine is cold or hot. It runs bad immediately from a cold start. Tach is working normally. No check engine light showing on dash. Things we have changed/checked

new:

-Cam sensor
-TPS
-Coolant temp sensor
-Maf cleaned
-Plugs/wires
-Coil pack
-Fuel filter
-Air filter cleaned
-02 sensors
-IAC valve

-no codes
-balancer fine

We drove the car with wideband lamda system throughout the rev/boost range and the a/f ratio kept very good all the time. No signs of lean rich anywhere. Guys at a local mustang shop where the car was for repair said they have no more ideas what to try with it.

Any ideas are appreciated! Sorry my bad english.


Thanks,
Aaro, Finland
 
Pressurize the intake at the throttle body (prop open the butterfly) with about 5 psi from an air compressor and spray some soapy water around all the vacuum fittings and anywhere else that may possibly leak air. I'd did this on my 94 SC which was missing at idle at the time and got bubbles from:
the throttle body idle adjust screw
one of the small vacuum lines on the vacuum tree
my Magnum Powers inlet plenum bypass valve shaft on both sides
a vacuum hose near the driver side firewall
And the grandfather of all vacuum leaks on this vacuum fitting (see pic) that had a hairline crack in it and was blowing air out like crazy. It's near the driver side firewall and you can see the brake fluid reservoir in the pic also. I put some Permatex gasket maker on it as a temporary fix and the car idled a whole lot better.

Dsc01499-1.jpg
 
did you remove the IC tubes when you did the plugs and wires, if so make sure you reseal the IC top and bottom really well, thats the spot i have trouble with

also the vacuum sounds kinda low if it is 10 at idle, so probably is a vacuum leak, use rtv or teflon gaskets to seal well
 
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And, like the post up a couple, I get 20 or more in/hg at idle. A leak in the plumbing may be your first solution.
 
re problem

when that happen to my 95 it seems everything cam,crank sensor, dis, coil, wires,spark plugs, vac line but ........ guess what it wasn't..... no check engine light on ....... no vac promblems .......... so i went under and removed the timing arround the balancer and saw my problem:mad: the balancer was broken but stil spinning .......but it was a hear line cracked and caused to move and miss firing ....... try to go under and look for cracks in the balancer shaft i almost bet that this is your problem..... ;)
 
Becerrasc, when your car had the balancer problem did it run fine at wot? Mine does and i don't think it could if the timing was off the mark, or could it?

Is the only way to find the vac leaks to pressurise the system or can they be found with the propane test also?
 
re problem

no it didn't the rpm seems to be good but the boost gage its the one that does not respond properly.....
 
get the car running and spray carb cleaner at the ic tubes and other areas of possible leaks and when the rpms pick up you have found your leak.
 
Wouldn't it show on a/f ratio on idle if I have a bad enough vac leak to cause bad idle/stalling?

I have mounts for wideband lambda system in downtubes and we drove the car with it. No signs of lean anywhere in the rpm/boost range. I ordered a vacuum leak tester from spinning wheels and will test for leaks when I get it but meanwhile was just wondering the question above.

If it was a balancer problem the car wouldn't run clean and strong wot to the redline would it?

I also ordered a new dis and an eec because we are running out of ideas on what to try with it next. The car is really bad to drive because of bad bucking in light throttle and stalling in allmost every traffic lights...:mad: Don't want to sell it broken because the price would be ridiculous and the problem might be very easy to fix at the end, hopefully...:D
 
Ummmm, maybe you should get an exorcist? It still sounds like a leak in the intake plumbing. Or a faulty vac/boost gauge?
 
Check for a cracked plug, sounds like your going though what I just did. Pull the plugs 1 by 1 and check for small cracks.

Jerry
 
Just out of curiosity, unplug your bap sensor, then start and run the car. You may get a cel but I'm just curious how the car will run.

Its located toward the pax side on the windshield cowls close to the throttle body. It'll have what looks like a vacuum connector on it... (don't plug vacuum into it.)

Fraser
 
Sounds like a vacuum leak. It doesn't take much of a leak to cause it to run like crap at idle and light throttle. I had one that wasn't bad enough to even register on the vacuum gauge (still showed 20 inches) but I could barely keep the car running.

I tried to find a leak for a couple days which was very difficult when the engine wouldn't stay running without constantly goosing the throttle. Frustrated I finally gave up and took it too the local Ford dealer...and while pulling it into the service department check in bay stumbling and sputtering....one of the techs came over and said "hey sounds like a vacuum leak go ahead and pop the hood". I did and before Icould even finish telling him "it isn't a vacuum leak because I already spent 4 over this hot motor hours searching for one", he starts spraying carb cleaner on the IC tube connections, and when he sprayed the upper IC tube connection it nearly died then quickly revved the motor. :eek:

Suggest getting a can of carb cleaner and spraying all the connections between the throttle body and intake manifold with the motor running. Start with the IC tube connections, because they are they most likely place for a leak. If you find a leak you can use copper RTV or order a set of teflon gaskets to reseal it.

David
 
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might this have anything to do with the EGR? egr turns off at wide open throttle, so that makes sense in your case, but if it's plugger, or faulty it might be causing issues at part throttle?
 
Fturner. I unplugged the Bap sensor and it ran just the same. Maybe a little worse but nothing significant. So can I count it out of the possible problem sources? I ran the car with it unplugged only car parked. Should i drive it with Bap unplugged, would it give any more info on if it's working or not

Cherry 93 sc. The plugs and wires have been changed like three times both so I don't think it's those eventhough I know they are a common source of problems with these cars.

Speedwagen. Could you explane a little bit more how could motor mounts or knock sensor cause these problems?

Has anyone had similar problems as my car caused by the egr system? Can it really mess up the cars running so bad that the car barely idles or keeps cruise speed highway without boost?

I'm waiting for my vacuum leak tester from spinning wheels and will test for vacuum leaks carefully next. Haven't found any with carb cleaner test.

Thanks for everybody so far. Troubleshooting continues...
 
Just ruling out the BAP sensor since those can go squirly and make the motor think its running in Jupiter but not throw a code for being out of range :). I'm with most folks here about a vacuum leak. All it takes is forgetting to hook up the vacuum line that runs from the intake tube over to the drivers side valve cover to get your symptoms.

Fraser
 
Hey, some progress but no solution... Tested for vacuum leaks with the spinning wheels tester with 10 psi and found no leaks. Changed the eec and dis and 02 sensors, no change...

The car is still stumbling bad at idle and also on throttle while on vacuum. Immediately when it goes the boost it starts to run clear and feels strong, no problems. It runs fine in wot, full boost, throughout the rev range. In vacuum, stumbling. It sounds like it's missing in idle. Plugs and wires have been changed a few times so it's hard to believe they would cause it.

Deleting the egr system next. After that have absolutely no idea what to try next...:mad:

When we monitored the o2 sensors with a scanner the other one wasn't switcing at all. Guys at the shop had some kind of explanation for it, but i'm thinking could it be the problem? Both o2 sensors have been changed and wires checked.

Could maf electronics cause this if the unit is bad.Could it run strong at wot and stumble at part throttle with a bad maf?

Thanks for any ideas!!!

It would be nice to get it running right now when I finally have the trans issues solved with a 03 mustang trans built by dirtydog Performance. Very happy with the new trans and converter, it's a totally different car to drive compared to with the worn stock one...:D


Aaro
 
Anybody! Some help needed, The car didn't pass the emissions test of inspection because of bad missing causing high emissions...:mad:
 
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