RPM drops in cyclical mannor whenever car feels like giving me a hard time

PART 1: I have a 2001 Ford Taurus SE with 97,000 miles. We bought it 3-4 years ago used and it seems to have always, or at least since August had an occasion problem with a slight drop in rpm that was cyclical in nature. Cyclical meaning it was once or twice a month for a few minutes or a couple times within a couple days. It was also cyclical in that when this occurred it doesn’t matter if you are in park, stepping on the the gas, or the break and it had a beat to it when it occurred. In August we couldn’t take it anymore because we noticed it was progressively beginning to interfere with the driving of the vehicle. We took it into a local Expert Tire shop when we began getting cylinder 6 misfire codes. They replaced the EGR Valve, PCV Valve, DPFE Sensor, all the spark plugs, tuned the electronic ignition, did a 3-step fuel system cleaner, and ran tests (like compression and other diagnostics and those were fine). We left town that day but not an hour out of town at 70mph the cars air conditioner went out, it made very low pitched banging noises (like someone taking a rubber mallet and banging on the engine) and a great shuddering rocking us in the car as we drove a mile more to the nearest exit. It was towed back to the original shop where they said the PCV Valve they replaced was defective and they replaced that. Yet, they couldn’t answer these questions at the time and maybe you can. Would a defective PCV or EGR valve or anything else make the air conditioning go out? What made the horrible shuddering (it shuddered when I applied the gas mainly but was always there)?

PART 2: All this work didn’t fix the initial problem and we made sure they knew that. So then began the monthly trips in for them to find holes in the vacuum, worn and cracked lines having to do with the air intake, and pen sized holes towards the end of December with none of these fixes fixing the initial problem. As this was going on we were sometimes coming in due to our check engine light which they said was coming on do to the wrong amount of air coming into the system. As our problems this past month have gotten much worse, this is the most recent and violent outburst our car has had: We were in Walmart for maybe 30min and upon starting the engine as we left the car repeatedly bounced from 800-1000rpm down to 200, we tried turning it off and back on with no change, we tried driving (more like jerking) with no change, we tried driving faster, slowing down, stopping at lights with no change. Gradually as it relented the check engine light comes on (later found to be a cylinder 6 misfire) as we got to our house a mile away. The next day, we take it to the shop, it acts fine the way there… They replace the Idle Air Control Motor (per my request) and as I pull away I notice the faint and then shuddering drops in rpm. The shop was never able to replicate what I was trying to explain so that may be why they don’t understand what is going on but it may also be that I found that only 1 person in there shop is qualified (well, at least they say he is) to understand this issue and I wasn’t told this until Jan. 3rd… Took it to Quick Lube because I was desperate and they were the only ones open. They managed to catch it in the act and were nice enough to let me watch as they read the reading on the check engine light and show me what might be happening after listening to my explanation. Without having a good look at the car or running diagnostics they have the idea that it is a malfunctioning Mass Air Flow Sensor or secondly (but he didn’t really think this as it would only be making the lag when I step on the gas) a Throttle Position Sensor… My question is, am I screwed? Could it really be this? What else could it we? Why is this so hard to fix? Why has it taken over a grand to TRY and figure this out? Has Expert Tire made a mistake because the past month they have been very apologetic and offered to fix all the vacuum leaks, try out the IAC motor, and do diagnostics for free just to get this fixed.

The computer is the brains of the operation. Sometimes it is insane. A bad computer can’t always tell you that it is nuts. Check the ductwork attached to the throttle body to be sure that it is tight and not letting cold air go right in. The inlet to the heat exchanger box (plenum box) may be off or broken.

What do you consider ‘cold air’? It was 52* yesterday and the car made the shuddering and rpm fluctuation. Also, the Mass Air Sensor has been ruled out as a cause as of today.

The place I took it to did not make note of some of the codes or the values on the compression test. These are the ones they did have. In August the car was brought in initially for p0303 and p0306 codes thought there may have been more. December 25th brought up bank 1 & 2 lean as it was shuddering (test was read on 26th). January 3rd was read for Random Misfire, Bank 1 & 2 Lean, and Bank 1 & 2 Misfire. Today, while driving with the diagnostic sensor attached the car shuddered (by about 200rpms) but nothing came up on the sensor and the readings appeared fine.

I would look for a possible intake manifold vacuum leak

The last 3 times I have ask them to look for a vacuum leak they find one. the last time I took it in they could only find a pen sized hole. Wouldn’t they have also checked the intake manifold or do i have to ask specificly?

If they never replaced the EGR valve could that be the issue? After doing some fine print reading I discovered they didn’t charge me for the part. All this time we were working on the assumption that it was replaced.

Took car to Ford and the issue turned out to be a big hole in the vacuume at the intake gaskets. From what I understand, it was at the connection with the O-ring and it was very obvious too. They stated that the other place I took it to should have spotted it. I also found that the loud noise I was describing (like a rubber mallot hitting the engine) was more than likely from the air (heating & cooling unit) opening and shutting due to the PCV Valve malfunction. Also, the antifreeze was brown so they flushed it, the drive belt was badly cracked so it was replaced, and it was time for the fuel filter to be replaced.

The only question left unanswered is how the antifreeze got so discolered? Would a hose on the vacume being disconnected for a time do that? If this sounds dumb, remember I know little about cars :slight_smile: