88 Cougar with 5.0 running rough

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88 Cougar with 5.0 running rough

December 05, 2012, 12:37:47 PM

My 88 Cougar with 5.0: Upgraded last year to HO intake manifold. Two weeks ago, I pulled the full dual exhaust from my other Cougar (Mustang exhaust, h-pipe, etc.) and had it installed at an exhaust shop. The car ran better than it ever has for about 2 days and then started running very rough at lower rpms, to the point of violent bucking if I don't floor it. After getting up to speed, it is not as noticeable, but still present. Fuel economy is now . I have had a vacuum leak for the past several months but the car ran fine with it before. I found a broken vacuum hose that goes from the front underside of the intake manifold to a canister on the passenger side fire wall, near the front. I fixed that this morning, but no improvement. The exhaust shop that did the work didn't install the nuts on the studs between the headers and h-pipe. I was able to buy some this morning and get the top nut tight on each header, but not the bottom. I
So, my questions are: Should I assume that there is a loose joint between the header and H-pipe and this is causing my problem? Is it possible that the connections I worked on/fixed this morning will take time to be recognized by the computer? If all other things are corrected, could the vacuum leak be causing the problem? Could a shorted or damaged O2 sensor wire be doing this?
Any suggestions would be helpful. Thanks.

88 Cougar with 5.0 running rough

Reply #1 – December 05, 2012, 12:58:30 PM

Finish up the HO mods. Get some E7 heads and an HO cam and pcm. Then switch the plug wires to the HO firing order.
Question, does HO intake swap mean fuel rail and injectors also? Your SO SD PCM probably won't play nice very long with oversized 19# injectors. Once it runs a while and trims out the fuel, you may very well get those sorts of problems. Too much air, too much fuel.

88 Cougar with 5.0 running rough

Reply #2 – December 06, 2012, 08:57:20 AM

Check for an exhaust leak upstream from the o2 sensors. When my header to Hpipe was leaking, it made my car run like and act screwy. That is where I would start, since that is where the shop was working.

88 Cougar with 5.0 running rough

Reply #3 – December 07, 2012, 08:35:46 AM

Have you pulled any codes?

Darren

88 Cougar with 5.0 running rough

Reply #4 – December 08, 2012, 11:06:33 AM

X2, pull codes.

In my experiance, and exhaust leak hasn't caused me any problems but noise. I have also never replaced an 02 sensor on one of these cars.not saying they don't go bad, I just haven't had to.

My money is on a intake or vacuum leak, which will make the car run super shiznitty. Especially if the intake has been off. If you unplug your map senor vac line while the car is running, does it get any different?

My old 87 had bad motor mounts, and would sometimes pull the vac line off the intake, the car would instantly start shooting out black smoke and economy would drop down to about 5mpg.

88 Cougar with 5.0 running rough

Reply #5 – December 08, 2012, 05:18:13 PM

I thought it was an exhaust leak between the headers and H-pipe because even though they seemed tight, the exhaust shop forgot the nuts. I added one nut per header in hopes it might help but no dice. So today I replaced the O2 sensor extension harnesses (one was kind of chewed up), still no dice. So, I figured I would worry about it later, or let a garage look at it. As I was starting to drive off, I remembered a small vacuum line I forgot to hook back up, and pulled over with the engine running to replace the plug. It was then I heard a tic-tic-tic kind of electrical sound, kind of like the battery powered igniter on a gas grill makes when you push the button. A little digging around and I noticed that one of the spark plug wires was dangling and in contact with the header, and the wire was shorting out every time that plug fired. A little re-routing of the plug wires and PROBLEM FIXED. Thanks to everybody that offered suggestions! And thanks a lot to the Meineke shop that did the so-so install.

88 Cougar with 5.0 running rough

Reply #6 – December 09, 2012, 08:47:58 PM

That sucks on the shop work but glad you got it fixed! I would start it back up this evening and with all the lights out look under the hood one more time to make sure you don't have any more issues.

Darren