1993 Camry - car shakes violently at low revs with or without A/C on, but especially with A/C on

Maintenance/Repairs
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My 1993 Camry has recently been giving me trouble starting today. For the record, I’ve had terrible fuel economy (15-17 MPG in all gentle city driving) for awhile now and have been trying to diagnose it for awhile and tinkering with the various things that could effect fuel economy and having my mechanic help me.

For the first time in several months the weather has been hot enough to warrant the A/C being on and I noticed immediately upon turning it on and my car getting up to temperature that the car would quite violently shake at idle, stopped, or at low speeds. I turned the A/C off and the shuttering was still there and the idle shakes were still rough, but not as bad.

The only thing I can do to get it to stop entirely is to put the car in park or turn the car off.

Engine mounts, transmission fluid flushing, new spark plugs (no new wires), battery check at NAPA (alternator is supposedly getting old and will need replacing soon but battery is fine), have all been done in the last month. Spark plugs were done last week as was battery testing.

I drove the car on the freeway up to about 70 MPH. The shuddering seemed to go away at about 55-60 MPH with overdrive on. With overdrive off I wouldn’t get shutter at any speed above about 40 MPH. When driving in traffic the car seems like it’s going to die, with throttle response being so intermittent (unless the car is at higher than, say, 2500 RPM) that it seems unsafe.

The car only has 78K miles and is driven almost exclusively in city traffic. I’ve considered that carbon buildup might be an issue but I’m not sure if it would cause these symptoms.

TL;DR, car shakes at nearly any speed at low RPM, gets really bad with A/C on, done some recent service that might point to it, and my city-only driving might have built up a lot of carbon. Throttle response is pretty pitiful too.

An edit: I cleaned out the throttle body which was gunked with carbon and the car ran better but still shakes like crazy, so I think the running better is unrelated.

Second edit: SOLVED IT. One spark plug wire wasn’t working properly. Replaced the wires and the car drives like new.

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Is a check engine light on?

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The check engine light is not on.

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Bumping, hopefully someone can help please!

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I own a Corolla of similar vintage. Here’s some ideas

  • read the diagnostic codes. there can still be codes stored in computer memory even without the cel warning lamp on. post the codes here for some idea.

  • egr is sticking on. as a test apply vacuum to the egr valve at warm idle. that should stall or nearly stall the engine. does it?

  • If the spark plug wires, distributor cap, and ignition rotor have never been replaced, that’s way past due, and could cause this symptom. check the ignition timing too.

  • look for vacuum lines that have fallen off, and test that both the map and brake booster hold vacuum to 20 inches

  • since the problem goes away beyond 40-50 mph, unlikely to be a problem with the fuel filter, engine air filter, or fuel pump.

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  1. Car doesn’t have OBD2. It’s from 1993. Not sure how I would have to test error codes.

2.Not sure how to do that but I will look into it. The car feels like it’s trying to stall when in drive and sitting idle, yes. Googling symptoms - this could be it. I have a strong feeling that carbon is built up in the car (since cleaning the throttle body made the car run a bit better) and could cause this.

  1. I mentioned in the original post that the plugs were just changed but I think the wires are about 3 years old according to maintenance records. They might be dead but I can’t say for certain. Ignition rotor hasn’t been done as far as I can tell based on the service history that I have for the car (back to 30k miles in 2005)

  2. Vacuum lines look good according to my mechanic when I first told him that the fuel economy seemed to be really awful about a month ago.

  3. Yep, and the higher the car revs, the smoother the driving experience is and the quicker the throttle response. I got the car up to about 4.5K while driving around in low gear and the throttle response felt normal at 3k+ RPM. Replaced air filter, fuel filter, and checked fuel pump all recently.

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It’s probably similar to how it is done on my Corolla. I have to place a jumper between two pins of the “check connector” in the engine compartment. The sticker on the under-side of the hood shows which pins. Then when I start the engine the codes appear as flashes on the check engine light. For example a code “23” would appear as you watched the cel as flash flash … pause … flash flash flash. To get the codes to repeat I press the gas pedal briefly. Most shops could do this for you and wouldn’t charge much for the service. Once you get the codes, folks here can help determine what they mean.

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I did mention before that the check engine light isn’t on. Should I still do it? I’m away from the car currently but can do it in a bit.

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Yes, there can be codes stored even with the CEL off.

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Have someone check the harmonic balance at the front of the engine to see if the rubber is coming apart.

Tester

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Alrighty, tested to see if there were any CEL codes and the light constantly flashed rapidly. From what I’ve read, it means that there are no codes. Probably irrelevant but my diagnostic port is covered in really gross yellow fluid that reminds me of earwax. Not sure what it is.

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Did you get it to flash “51”? On my Corolla it flashes constantly like that (when the check connector jumper is installed) until I start the engine and press on the gas pedal. Once it starts to idle, then each time I press on the pedal it will stop flashing constantly on/off/on/off and will instead flash a code “51” then other codes (if there are any stored). But it always flashes code “51” when I press the gas pedal, which means the throttle switch is working, and the code read-out procedure is working. From what I see your car works the same way. Do you see the code “51” flash out when you briefly press on the gas pedal in idle?

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Nah, just a steady flash of the CEL.

I edited the original post as I fixed the problem. Feels kinda dumb but I guess that the spark plug wires were original to the car. Replaced them and I literally have never felt the car drive this smoothly before. Hilarious but I probably should have just replaced them when I replaced the plugs!

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Congrats. I had a spark plug wire arc’ing on the my truck a while ago and causing it to idle roughly, and my truck is a v8. Your car has fewer cylinders, so it is more sensitive to one plug not firing consistently. The dizzy cap and ignition rotor should replaced also, imho, if that hasn’t been done recently. On my Corolla the spark plug wires and dizzy cap is one ass’y, so if the wires are replaced, the cap is automatically replaced. Replacing the rotor is usually a doodle when done as part of that job. It’s a good idea if you plan to diy’er fix this car to learn how to work that diagnostic code system. It can be quite helpful, and safe you a lot of time looking for problems. For some reason the service manual has a really difficult time trying to explain in simple terms how to do it. Go figure.