Serious miss at high boost

dklitzke

Registered User
:confused:

I have a 1995 SC 5-speed that currently has a little over 27k on it. I bought almost two years ago with right at 22k on it. Since then, I had a full (not including headers) SCP exhaust installed. Then I installed a ported MPII SC that I had previously bought for my other SC (automatic-currently 188k). Installation went well. Then early this year I bought and installed a BBK 75mm TB, a 76mm C&L MAF, and the SCP air intake setup. Somehow, it has developed a serious miss at high boost. Other than that, it seems to run fine, but it's frustrating when you want to open it up, to have it start coughing like a sick puppy. It still has the original plugs and wires, but they should be okay given their low mileage. However, once my broken wrist heals and I can work with both hands again, I will probably replace them. Any ideas on what's causing the miss? I only run premium gas. Could it be a vacuum leak? I am sure everything is sealed proper, but maybe something's leaking? Any help you guys might have is appreciated in advance.

Don
Prospect TN
:rolleyes:
 
The only time My sc ever acted like that was when I changed my plugs/wires and got a bad wire from the factory. Basically ended up doing the whole thing twice. It would run nice and smooth until I really got on it then it was choke and cough city. I would double check all the vac lines and pvc line but you've probably already checked that. The plug gap is a good idea. Might have been fine before all the extra air was introduced but now one or more of the plugs that might have been gapped slightly larger than standard is getting the spark blown out by all that extra air. I would put new motorcraft plugs and wires in and set the gap at the stock 0.054 or slightly smaller due to the extra goodies you've got on it.
 
drop the plugs 1 heat range to an autolite 764 and gap .040-.045, solved my problem last year. Might also try a set of wires, may have low mileage but they are 12 years old now.
 
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Miss under boost

Sounds like an ignition related problem. Premature spark extinguish, or no/weak spark under load.
 
My stock 91 SC was doing EXACTLY the same thing. I figured plugs and wires (90k miles). Turned out to be a major vacuum leak from the PCV tube. Ran great after it was all back together.

Jason
 
Have you by any chance upgraded the injectors, and a bigger fuel pump?

Thats ALOT of air moving into that motor but there's nothing been done about fuel delivery to keep up (that you've mentioned here)

Also make sure you have the right sample tube with that MAF.
 
Hey, thanks youse guys!

:D

Like I said in my post, broke wrist prevents me from doing much right now, but I will do the plugs and wires next. To answer the last question, no I haven't upgraded the fuel system yet, but u r right probly need to. I will also check the PCV to make sure it ain't a leaking.

Don
 
When mine was missing at high boost, it took me awhile to figure it out but it was a clogged cat. If your pushing more boost then what you think you should be getting and if your engine is running alittle bit warm, check that!
 
When mine was missing at high boost, it took me awhile to figure it out but it was a clogged cat. If your pushing more boost then what you think you should be getting and if your engine is running alittle bit warm, check that!

I thought that too, then I realized I didnt have any catalytic converters...

Plugs Wires & Plugs Regapping (3 out of 6) helped MAJORLY, but it still didn't solve my overboost problem, and I even dropped all of my exhaust checking for restrictions and driving around without for a little...notta

I've narrowed it down to bad boost gauge I guess, need to test it out.
 
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I went through this EXACT same thing not that long ago. I thought plugs as well. When I went to chnge them, I found a wire that had a broken terminal on the plug end that was allow it to not hook to the plug. After replacing all the wires and plug, it was only just slightly better. I put a Fuel pressure gauge on it and found that it was not holding pressure when the key was off. I pulled the tank and took out the pump and found that the 2"-3" hose the connects the pump to the hard fuel line had a crack in it. It would run fine around town all day, but as soon as it made over 5psi, it would sputter and cough. As the fuel pressure would increase, it would bypass it out of the tear and not supply enough to the engine. If you haven't upgraded your pump yet, you may as well as you have it out.
 
If you have never done your plugs then its best you do..I have seen SC's with really bad examples of what plugs should look like. The best is when only 4 or 5 have ever been changed with one original still in their..

Also get yourself some wires because ther eis nothing like going to do your plugs and a wire ripping on you
 
exhaust leak?

My old sc used to miss under boost. turns out there was an exhaust leak where my downtubes met the resonator. hope this helps because that damn miss drove me CRAZY lol.


Paul
 
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