Shudder at idle - misfire engine codes don't diagnose problem accurately

SL-2 engine replaced at 169,000 saturn program engine installed w/new motor mounts,has new idle control circuit, clean intake & throttle plate, new coil pack on #1&4 cylinder, New plugs, new plug wires, 231,000 miles on odo and leaving dealer for engine code repair “random misfire cylinder 1 & 4” engine light comes on again (5th time from leaving service!!!) when coming down from speed and engine is about to idle engine idle shudders and dash and door panels shake and vibrate/resonate, motor mounts are tight even that funny little bar mount that attaches to the underside of the intake manifold; Trans has 231,000 original miles have had the tsb valve body reverse slam repair, trans has difficulty shifting into higher gear tends to wind up in rpm until throttle is closed slightly then it shifts

Misfire code has me stumped…what sensor needs to be checked?..did I miss a TSB in earlier years…

You’ve gotten a lot miles out of this car. It’s a good car but there is such a thing as an undiagnosable cluster of errors. I think you may be at the point where the monthly payment dumped into a much younger used car is going to cost you less that the significant repairs this car is about to cost you.
The transmission is going. The engine has two cylinders misfiring. The car has 231,000 miles on it. It is time for a new car or at least a newer one. Buy a Toyota used or another Saturn (how this car brand never caught on is beyond me, they are terrific cars, better than any other GM brands).

The first step when an engine performance problem exists is to check the compression.

I’m not familiar with the engine program you refer to. Are you saying that Saturn gave you a brand new engine at that obscenely high mileage or the dealer installed a new to the car engine which in actuality is a used one, etc.?

Just trying to clarify this a bit.

More clarity is absolutely necessary. Try full sentences complete with punctuation. Also clearly lay things out in time. I guess your engine currently has about 60K on it?

“random misfire cylinder 1 & 4” doesn’t refer to anything - there is no such code.

There is a P0300 - which is a random/multiple cylinder misfire.
There is a P0301 - which is a cylinder 1 misfire
And there is a P0304 - which is cylinder 4.

Clarify & specify the codes.

As OK noted, checking compression is best to do first to rule out a mechanical issue with the engine.

If that looks ok then check out the fuel system. Have you checked the fuel pressure? Operation of the fuel injectors? Mis-“fire” is a little bit of a misnomer b/c it doesn’t refer exclusively to a lack of fire (spark). Its actually more like mis-“combustion” - as in the air/fuel mixture didn’t ignite. Causes? Inadequate/no spark. Poor compression. Too little or too much fuel. Too little or too much air.

If you do determine that there is actually a spark problem don’t rule out a damaged PCM. You should also check the crank and camshaft position sensors.

The car was purchased new in 1996 and I’m the original owner. At 164,000 miles I noticed the inside of the oil fill cap was getting peanut butter/chocolate milkshake colored. A few days later the coolant tank level sensor went on and I noticed the tank was dry. I filled it and the tank went dry again several days later (no coolant leaks or bad hoses anywhere). So figuring I had blown a base gasket it was time for a new engine. Great car so far… 35 mpg on highway. Saturn dealership would give a discount credit for an engine still running so I took it in and had a Saturn Program engine installed with a Saturn warranty. This was a rebuilt engine from Saturn.

Starting at 206,000 miles or 42,000 miles after the engine was installed I started getting the following engine codes showing up:

PO300, PO301, PO304

The most frequent code is PO300 - random cylinder misfire

Current mileage on the 2nd engine is 67,000.

I had the throttle plate/intake system cleaned last week. Less than a mile away from the dealer the engine light came on at 75mph. I disconnected the battery to clear the code and at 75mph the engine light comes back on. If I don’t hit 75mph or accelerate to pass and get up to 75mph the engine light never comes on.

I will compression check the engine. I burn about 1 quart of oil every 2,500 miles

I suspect the fuel injectors are original to the car as I doubt whether I got new ones with an engine install at 164,000. I have yearly put a fuel injector cleaner in the fuel tank and run the tank down.

I cannot afford a new car payment and this is my only driver. It gets driven like a taxi as in lots of engine starts and stops, surface street and freeway driving. I’m driving about 2,000 miles a month. I am meticulous about fluids, servicing, and preventive maintenance. It gets all fluids and filters changed yearly (oil/filter every 2,000)

I’m willing to check fuel pressure and fuel injectors.

Is there a DIY procedure for checking if you have a bad fuel injector?

Thanks for the advice. Any more is greatly appreciated

Ahhhh…much more clear.

Do you suppose its a 75mph thing, or an engine load thing? I’m thinking that if you left it a gear lower and drove it at something like 50 (or whatever would produce the same rpm/engine load) you might get the same thing.

I’m still thinking fuel delivery problem. How old is that fuel filter? I’d get a gauge on it before messing around with the injectors. You’ll need to get a read on it while it is under load, not just a static pressure reading.

Since P0301 and P0304 mean that there was no spark on those cylinders for some reason, and given that the available power to generate a spark goes down as RPM increases and that cylinder 1 and 4 are fed by the same coil, I’d look into a new coil.

I see that you did replace that coil once, if that was after this problem started and it didn’t fix it, I’d still look into that circuit, there is more than just the coil. You have to look into the primary wires that send the current to charge the coil. A connection that has gotten some corrosion on the pins or is getting a higher resistance than it used to get could limit the current to the primary of the coil and reduce the available spark.

I believe the primary current comes from the computer, so check those connections too. Inside the computer, there should be two large power transistors that act as switches to control the current in the primary circuit, possibly one of those has developed a problem.

Brilliant! I never considered connections under covers or protected by insulation might have corrosion and cause resistance. Forgot about the stuff out of sight. I’ll get on it:)

I’m not telling you not to check that stuff out. But those codes don’t mean that there was a lack of spark. It really does just mean that there was a lack of combustion. (Although even more accurately I think it means that the crank position sensor detected a anomaly in the speed of crankshaft rotation but don’t quote me on that one). This might have been from a lack of spark…or compression…or poor fuel/air mix.

This might help:

I’d do much more basic things like check compression and fuel pressure before digging into the wiring - though at least checking the voltage supply to the coil is easy.