'94 ETC Random A/C shutoff

[Ion]

The plug wires, the spark plugs, the FPR, a dirty throttle body, vacuum leaks, clogged air cleaner, and coils, in that order. Others will have their inputs too.

Note that anything that causes a miss will eventually throw a code, as will a vacuum leak. A clogged air cleaner can throw the same codes as a vacuum leak.

How does one test the coil packs?

[Cadillac Jim]

The simplest way is to remove the coils from the packs and check the primary and secondary resistance, but I wouldn't do that until I had gone through the other things. Note that I put coils last on the list. That's because they don't go bad often and are the least likely to be causing the problem, particularly since you don't have any codes. And, they are quite hard to get out and test relative to all the other items.

I believe that earlier on you had some MAP codes that looked like a bad connection to the wiring harness. This can definitely cause driveability problems. Have you fixed that yet?

If the MAP connector is clean and tight and the MAP codes have gone away, and you have no PCM codes but still very noticeable livability problems, the most likely culprits are the FPR, a dirty throttle body, clogged air cleaner, vacuum leaks.

[Ion]

The simplest way is to remove the coils from the packs and check the primary and secondary resistance, but I wouldn't do that until I had gone through the other things. Note that I put coils last on the list. That's because they don't go bad often and are the least likely to be causing the problem, particularly since you don't have any codes. And, they are quite hard to get out and test relative to all the other items.

I believe that earlier on you had some MAP codes that looked like a bad connection to the wiring harness. This can definitely cause driveability problems. Have you fixed that yet?

If the MAP connector is clean and tight and the MAP codes have gone away, and you have no PCM codes but still very noticeable livability problems, the most likely culprits are the FPR, a dirty throttle body, clogged air cleaner, vacuum leaks.

No MAP error codes since installing new one. No codes at all, except for suspension-related nonsense (replaced air struts long ago). In fact , no "P" codes exist at all. Wouldn't a bad FPR throw a relevant code if it was malfunctioning? My engine runs choppy consistently...rather than a steady purr, when it's running correctly, its now more like a constant "bub, bub, bub, bub..." I'll def. look into the remaining areas.

[Cadillac Jim]

The FPR will not throw a code. It's a Bosch analog part that maintains constant fuel pressure across the injectors by using a spring (which sets the pressure differential at about 44 psi) and a vacuum diaphragm (which uses manifold vacuum to buck the spring and decrease fuel rail pressure as vacuum increases). With no sensors for fuel pressure, there is no way for the PCM to check on the FPR.

The most common failure of the FPR is a leaking diaphragm, which allows gas from the fuel rail to leak into the vacuum line. When the car is shut off, the fuel rail pressure bleeds gas into the throttle body which makes for a very rich mixture on start-up, which can cause slow starting with a hot engine, a first sign of FPR failure. If the leak is bad enough, it can disrupt the fuel mixture by forcing it to be too rich while the car is running, particularly at idle, and an OBD II system may pick that up and throw a code but not an OBD I system.

You can check an FPR easily enough by pulling the beauty cover (a bit more of a hassle on the 1993-1994 than later Northstars) and pulling off the vacuum line and seeing if it smells of gasoline, or is wet with gasoline. If the vacuum line does not smell of gasoline and is dry, the FPR is probably good.

The "bub, bub, bub, bub..." sounds like a consistent miss by one cylinder to me. If you don't know when your plugs and/or plug wires were last changed, that is the first thing to do. It is likely to cure your problem and in any case you will likely want new plugs and wires to ensure smooth running after any further repairs. Be sure and be careful with the routing of the plug wires - a mislaid plug wire can cause cross-firing or arcing under the beauty cover and even ruin your new plug wires.