Problems, Problems, Problems by Ray Paulk

My wife is always getting after me for working on the cars so much. But it only seems that she starts into this when I can finally get back to my 88 GT, Midas Touch, to play. The majority of time I spend is fixing one of our other cars or my kids cars. I have to point out that my car has been the most reliable of all our cars. With my car, I'm usually making upgrades or detailing. This is work for fun. Well, this sanctimonious position came crashing down recently with a series of concurrent or cascading problems.

I have been fighting a poor performance problem with my car for a while now. It started with an odd problem. Seems that when the tank got to about 3/8ths, the car would start to run like it was running out of gas, spitting and sputtering along. In trying to get the car home under its own power, I heard a howl like the "moan" you can sometimes get from your brakes. It seemed to be coming from the center tunnel of the car. It was finally resolved that as the fuel level dropped in the tank, it started exposing the pump itself to the air above the gas. One purpose of the pump in the tank is that the gas also cools the pump. With the pump partially in the air, it was no longer cooled properly and would gradually heat up. The hotter pump would not pump the gas properly starving the engine. This was fixed with a new fuel pump. Problem #1 solved.


Problem #2:

With the fuel pump fixed, I could drive the car, but it didn't seem to have the power it should. Plus, every once in a while it would stumble. I also found that at a constant speed, the car would pulsate, a cyclic speed-up and slow down, about once per second. In reading the GM factory manual, I found this condition to be called "chuggle". I tested, swapped, replaced and evaluated everything that the book said it could be to no avail. I also threw in some injector cleaner into the fuel but that didn't seem to make any change at the time.

I hit the TechNet with my symptoms and got lots of responses but nothing that I hadn't done, several times. That is until Gunar Dzintars asked if my car was an automatic. It is, but what does this have to do with chuggling. Gunar said that his car did the same thing after he installed a Transgo shift kit. The president of Transgo finally suggested that the TCC (Torque Converter Clutch) solenoid can, on rare occasions, pulse on and off like a very slow chatter. This gives a chuggling feel as the tranny shifts between lock-up and unlock. Since this solenoid and the 3rd gear switch are both inside the tranny, I just unplugged the connector to the case. Sure enough, the chuggle disappeared. I'm glad that Gunar came through 'cause I would have never come up with that one. Problem #2 identified!


Problem #3:

Now we're down to just poor performance and a stumble every once in a while. With a couple other problems gone, I could tell that the car was not running smoothly. When I diagnose an engine, I work through the Otto cycle, intake, compression, ignition and exhaust. Rough running is generally an ignition problem but I'd already replaced all of those components. When I installed my Jacobs ignition, I put in an extra plug so that I could easily swap back to the stock system. Since I also have the use of my son's Formula, I swapped the entire ignition system over to my car, eventually even the distributor. Since the Formula runs very strong, I knew the components were OK but my car still ran rough with periodic stumbles.

When I was checking the timing, I noticed that the car ran better in the test mode (jumping the a-b terminals in the ALDL connector). This implies that the ECM was either not working properly or some sensor input was not right. But I have an advantage most Fiero owners don't have. I have a DashScan code scanner built into my car. I can read all the codes and monitor sensor inputs at any time. Unfortunately, all the scan info was perfect.

After checking the timing, which was right where it should have been, I pulled out the ALDL jumper and restarted in the normal running condition. It ran rough for a few turns but then, all of a sudden, it smoothed out. All I can figure is that there may have been some dirt in one or more injectors. The combo of the injector cleaner and running in the open loop test mode, must have dislodged the dirt. I hate these mystical cures but will take them. Problem #3 solved.


Problem #4: Now the car runs smooth but still has no power

What can it be now. I'm down to just injectors and exhaust. In checking the factory manual, they suggest you use an exhaust back pressure gauge which screws into the O2 sensor port. The Factory manual says the at idle, with the engine at normal operating temp, the back pressure should not exceed 1-1/4 psi and, it should never exceed 3 psi over the range.

I ran over to AutoZone to pick up an exhaust back pressure gauge. They never heard of it! I checked with the mechanic who sometimes works on my other cars. He never heard of it. I called Midas and Meineke, they've heard of them but don't use 'em. You might think I had three heads when I asked for one.

In lieu of not having a gauge, I thought of (and several people suggested) that I unbolt the exhaust system from the manifold and see if the performance improved. Trouble is that the noise of an open exhaust under hard acceleration would not make me a popular neighbor. Plus, being an Engineer, I like quantitative evaluations. Measuring the back pressure would give these results. I just had to figure out how to do it.

All I needed was an exhaust back pressure gauge. Since I couldn't readily buy one, I had to adapt or make up something.


Exhaust Back Pressure Gauge

Having searched several auto parts stores for an exhaust back pressure gauge, I knew that a standard vacuum gauge could also read pressure up to 10 psi. This expands the use of the gauge so it can also measure the fuel pressure on carburated engines which is usually around 5 psi. It's also in the range I'm looking for to measure back pressure. The only killer would be the heat. A fuel pressure gauge could also possibly be adapted to measure the exhaust back pressure but due to its higher pressure range, the graduations were rather coarse if I was only looking for a few psi.

I decided that if I could make some sort of adapter to keep the heat from the gauge, I could probably use the vacuum/low pressure gauge. To give VISA a break, I bought both a vacuum gauge and a fuel pressure gauge at Sears. I got both not knowing if the back pressure might exceed the 10 psi limit of the vacuum gauge. Note that one contributor to the TechNet wrote that he had measured the back pressure on his car and measured up to 21 psi. I had to be prepared for this. I also made sure that I could return either if not opened. One reason I got the Sears one is that the vacuum gauge had a brass barb tube fitting. Others I found were plastic. Now all I needed was an adapter to get the pressure from the O2 sensor port to the gauge.


My Exhaust Back Pressure Gauge Adapter

First thing I did was pull the O2 sensor and measure the thread. Its an M18 x 1.5 metric thread. My idea was to make some adapter to screw into this hole and adapt over to a barb fitting through a thermal insulator. The first step was the threaded part. Problem is that I don't have a machine shop and I don't have a M18 die of any thread pitch.

In poking around AutoZone, I noticed a set of spark plug re-threading tools. They fit both 14 mm and 18 mm threads. Aha, an 18 mm spark plug just may fit the O2 sensor port. If so, I could break out the ceramic and make an adapter. Next stop was Wal Mart for some cheap plugs. I got a pair of Autolite #46 plugs for $1.94. At home, I cut off the steel contact with my Dremel cut-off wheel. Then with a prick punch and after ruining several drill bits, I finally got all the ceramic out. Seems that the top side of the plug has about a 9/16" bore and the bottom side has about a 7/16" bore. These happen to be the tap drill sizes for a 3/8" and 1/4" NPT thread. I first went with the 3/8" NPT tap but bottomed out too soon. I then cut a nice 1/4" NPT thread.

The idea was to go from the pipe fitting to a short piece of 1/4" OD hard nylon tubing which I had (for insulating) and then to a barb fitting. I used the nylon because it has high melting and softening points and good thermal insulating properties. It was then off to Home Depot for the parts. A 1/4" pipe nipple, 2" long, (2) 1/4" FNPT to 1/4" OD tube compression fittings and (1) 1/4" MNPT x 1/8" barb fittings. Assembling these with about a 6" long piece of the nylon tubing gave me my adapter.


The Tests and Results

Since there's no reason to press my luck with the heat, I did all the tests with a cold engine. I started with my GT. At a fast idle, mine ran 2-1/2 psi and at 3000 RPM it was 7 psi. I then tested the Formula, at idle ~0 psi, at 3000 RPM, the same, ~0 psi. Kinda looks like there might be a problem with the exhaust. Only thing now is to figure if it is the cat or muffler.

The way to check to differentiate whether the problem is in the cat or the muffler is to take a pressure measurement between them. I drilled and tapped a 1/8" NPT fitting into the straight section of exhaust pipe which passes under the engine. I threaded in a 1/8 MNPT to 1/4" OD tube fitting. This allowed me to use the nylon tube portion of the adapter previously made for the O2 sensor port adapter.

I made measurements at this point at idle and higher RPMs. There was no discernible pressure. After the test I had to blow into the tube on the gauge to make sure it was working. (It was).

I can say conclusively now that my cat is bad and the Fiero Store Ocelot muffler system is fine. As Dan Perry of the TechNet suggested, perhaps during all this my car was running too rich which gave me a bit of a melt down of the cat. Problem #4 is identified!

I want to thank all the TechNet people for pitching in and offering suggestions as to what the problem(s) might be and how to solve them. I know that a lot of you were following my progress and scratching your heads too. I was fortunate to have an additional Fiero which was running strong so I could test the various components. I have to say that I have never seen or heard of so many compounded problems. The diagnosis is now complete. All I have to do is recover from buying all the things like the timing light, vacuum gauge, plugs, pickup coil and the like, so I can afford the new cat, TCC solenoid and 3rd gear switch. But at least I have direction.

The worst part for me is that I can't brag to my wife anymore about nothing ever going wrong with my car!

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