Shadetree mechanics, lend me yr ear
November 19, 2007 3:56 PM   Subscribe

Gearheads, please help with a diagnosis!

2001 Hyundai Elantra GL 2.0 Automatic

Car runs a little rough (not too noticable, except when I'm in the engine compartment). When I put it in gear, though, the whole car starts to shudder and shimmy, even though I'm not moving at all. When I do try to accelerate the check engine light begins to flash and there is almost no power to accelerate at all. This problem seemed to come out of the blue--car ran fine in the AM and then started this when I tried to go to lunch. I've replaced the pcv valve, put new plug wires on it. I suspect there may be a vacuum leak somewhere, but I'll be damned if I can find it. Engine code reader tells me:

300 Multiple misfires detected
705 Inhibitor (Park/Neutral) Switch Circuit Malfunction

The car has no egr valve (at least thats what autozone and Chiltons tells me) so that can't be it. I just got gas the other day, but I've never seen such a violent and debilitating reaction to bad gas in a car, so I tend to dismiss that as well. New plugs were put in this car about 5000 miles ago (86000 miles on the car total), as well as a new thermostat.

Any ideas what would make the car do this? If there is a vacuum leak and I can't find it by feel, is there any trick to doing so? All advice accepted at this point.

The internets, when it comes to car troubleshooting and decent information, has been terrible.
posted by Chrischris to Travel & Transportation (10 answers total)
Test the ignition coil(s), check your vacuum with a vacuum gauge and try adjusting the neutral switch. You can test one of these with a multimeter but you'll need to work out the pins or get hold of a diagram for it. I would think the car wouldn't start with a bad neutral switch, but the PCM may use it for more than just inhibiting the starter while in gear. A quick test is to put the car in gear and turn the key, if it lurches forward the switch is bad (try in all gears).
posted by IronLizard at 4:10 PM on November 19, 2007

If there is a vacuum leak and I can't find it by feel, is there any trick to doing so?

This isn't safe, and you may kill yourself, etc, etc, but you can unplug the o2 sensor, idle the car (in park!), and slowly move an (unlit!) propane torch along the vacuum lines. if/when your idle changes, you know you're in the neighborhood.

Also, make sure your plug wires are correctly routed, your distributor cap isn't arced, and that your fuel injectors are properly seated.

Drygas costs $1.99 or thereabouts. You may want to give it a shot just in case bad gas is the culprit.
posted by Kwantsar at 4:11 PM on November 19, 2007

The two issues don't have to be linked. The Switch Circuit Malfunction could have been there for a long time, and just not created any symptomatic issues.

The misfiring is almost definitely where your lurching, shuddering, and check engine light are coming from. The source of the misfire is a toughy. Have you driven the car any substantive distance in this state? What is the exhaust gas like?

This is not a recommended troubleshooting plan (at all, in fact I am not really suggesting you do it). But... going through, 1by1, pulling 1 plug cable at a time might help you to find the issue. It would separate ignition issues from plug/cylinder issues. If you find that your issue is cylinder specific then you might want to investigate your compression in that cylinder. I had a similar problem at one point, and mine was a tiny head gasket crack.

Best of luck man!
posted by milqman at 4:28 PM on November 19, 2007

I had this exact problem in my Integra, twice, and both times I had burned (i.e. destroyed) an exhaust valve. After the second time, I replaced the engine. Pray that this is not your problem, but be aware that it might be.
posted by autojack at 4:29 PM on November 19, 2007

OK, it runs rough, but is it hard to start?

There are a lot of conditions that can cause this, including unpleasant things like a blown or leaky head gasket. Keep an open mind as you explore the root causes, and if you feel out of your league, take it to a pro.

Since you feel there's a vacuum leak somewhere, here's more info on vacuum leaks:

You can also try using a can of compressed air, such as you'd use to blow dust from a computer, and spray this around the intake track (NOT into the air filter, but over the parts that should be sealed). Wear safety goggles/glasses to protect your eyes from debris.
posted by mosk at 5:25 PM on November 19, 2007

I am NOT a Hyundai expert, all I did was some googling,

found this while searching... Hyundai P0300

This guy seems to think possibly your crank sensor is loose or bad.
--- cut-n-paste from the page I linked to...
"P0300, P0301, and P0302 are all misfire codes (cylinders #1 and #2, and a multiple/random misfire). The only time I've heard of this being related to a crank sensor is if the sensor were loose. Furthermore, a crank sensor issue should affect all cylinders equally. So I'd expect you'd have the P0300 only, or all the codes you have plus P0303 and P0304 (and P0305 and P0306 if a V6)............""

Other web sites suggest it could simply be really bad gas, water in the gas, or bad fuel pump. The fact that your getting the P0300 code telling us all the cylinders are missfiring is a good hint that its something effecting all the cylinders, (fuel pump, crank sensor, bad gas, vacuum leak like others suggested, etc etc))

I suggest check your engine mounts, look at them, see if they look torn up, that would cause the engine to shake more when in gear, but doesn't explain lack of power that you report. Im just thinking out loud here..

Wish I could be of more help.
posted by BillsR100 at 6:02 PM on November 19, 2007

It's quite probable that your fuel system, including fuel, is the cause of a sudden onset, multi-cylinder mis-fire condition, particularly one bad enough that there is barely any net power for acceleration. A vacuum leak severe enough to cause that level of mis-fire wouldn't be subtle, or hard to find.

But low fuel pressure (from clogged fuel filters, or a failed fuel pump or fuel pressure regulator) or a clogged evaporative emissions control canister vent, certainly could. You'd need to connect a fuel pressure guage, and verify that the fuel pressure and volume delivered are within specs, and if not, take appropriate remedial action (drain contaminated fuel, replace filters, fuel pump, etc).

Dark side possibility: If you haven't had the timing belt replaced (60K mile maintenance interval, usually), it might be that you've jumped a tooth on a worn belt. If you suspect that could be the cause, don't run the engine more until the belt has been replaced, and proper valve timing restored.
posted by paulsc at 6:17 PM on November 19, 2007

So... Spark plugs are basically new. Compression test isn't going to help because the problem isn't localized to a single cylinder (error code 300 means "all the cylinders;" individual cylinder misfires are the last digit of the 300 errors--301 would be the first cylinder, 302 the second, etc.) You don't mention any coolant loss, so it's probably not a blown head gasket. Your fuel injectors could be fouled, or the fuel pump could be on its way out.

I'd put money on a vacuum leak or fuel pressure regulator problem.
posted by Civil_Disobedient at 9:14 PM on November 19, 2007

Response by poster: Update:

per my father-in-law, I threw some drygas in the car to see if that helped. It didn't.

The car is idling very rough now, but the engine responds when accelerating by revving very smoothly. Its only in the lower rpm (750 and below) that there is a kind of hitching and then it idles rough. I pulled the plugs and took a look. Plug 1 and 4 (which share the same ignition module in this car) are pretty fouled, but 2 & 3 look ok. I'll replace the plugs (and look into an ignition module if that doesn't help), but I still suspect that there is something else wrong. The shimmying while in gear is still there, but no worse now than it is while the car is idling, it seems. TRhe car will accelerate and drive, but nearly always stalls when I come down to idling speed.

Any further thoughts?
posted by Chrischris at 9:34 AM on November 20, 2007

TRhe car will accelerate and drive, but nearly always stalls when I come down to idling speed.

I used to have something like that happen with my Miata--they called it "idle droop". There were a couple of causes and solutions. The first potential cause was the spark plug wires (not the plugs... the wires the plugs attach to). These only last about 50k miles before you need to replace them (at least, on the Miata they did... YMMV).

The second potential cause was a clogged up intake manifold.

The third was an incorrectly-set ECU idle speed. On the Miata this was a cinch to fix, but I don't know if your car has end-user-access to bypass the ECU.
posted by Civil_Disobedient at 5:10 AM on November 21, 2007

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