My car is lurching
July 28, 2010 4:46 PM   Subscribe

Why is my car engine surging only on hot (above 90) days?

Recently my car, a 96' Subaru, started surging on hot days - say above 90F or so. Usually this happens when the RPMs get above 2000ish. It usually seems most severe in 3rd gear for some reason.

The surging feels like I am pumping the throttle, and the car does this herky-jerky acceleration as I get up to speed. Typically once I get up to 5th gear it smooths out.

Any ideas? My coolant levels seem fine, and oil levels are ok.

posted by pilibeen to Travel & Transportation (9 answers total)
That sounds like what my cracked headgasket felt like.
posted by mumkin at 5:02 PM on July 28, 2010

I would suspect a bad fuel pump or a bad fuel relay switch. And intermittenant surging problem though is most likely not enough fuel and the fuel pump relay is pretty easy to replace in most cars. I would just replace that and see if it clears up. If it doesn't, a fuel pressure test would be where I would go next on my car and see what it tells me. It could be lots of other things as well though, and weird behaviour like this can be a bear to track down.
posted by bartonlong at 5:04 PM on July 28, 2010

++ on the relays, etc.

Relays, coil packs, and ICU's are temperature sensitive when they start to go, and are a real bitch to track down unless you've had one or two go bad on you and know what it feels like.

I'd go to a mechanic and pay the $30 to have them run basic diagnostics on the computer. If everything checks out, I'd put money on a bad relay.

Or you could just spend the 12 dollars or so and replace them and see if it clears up.
posted by onedarkride at 5:23 PM on July 28, 2010

Hot days tend to amplify the already hot conditions in the engine compartment, so I would first check things located there that can lead to stumbling like ignition coils/modules and airflow sensors. The fact that it tends to be correlated with higher RPMs also makes me think ignition. The fuel pump is not in the engine compartment and is cooled by the gasoline in the tank so that sounds pretty improbable to me but the fuel pump relay is certainly a candidate. I'm assuming that there's a temperature gauge in the dash and it's not showing overheating? If it was then you'd definitely want to check that the fans are working and that the thermostat is opening.
posted by Rhomboid at 5:38 PM on July 28, 2010

Automatic transmission or manual transmission?
Maybe a slipping clutch or low fluid in the automatic transmission?
Does the tachometer surge in phase or out of phase with the acceleration?
Does the engine race when it is in the deceleration phase of surging, or does it bog?
posted by the Real Dan at 5:46 PM on July 28, 2010

How long has it been since you replaced your spark plugs and wires? I had a similar issue that ended up being coils (expensive!) and my roommate had the same symptoms and cured it with new plugs and wires (cheap!).

If you are somewhat mechanically competent you should be able to replace the plugs and wires yourself. The hardest part is getting to the plugs themselves.
posted by thekiltedwonder at 7:37 PM on July 28, 2010

I imagine your Check Engine Light is in, right? It's rare that a car made since 1996 would have those symptoms and not store a fault code. There could be a number of things wrong and no silver bullet obvious answer. There could be a faulty sensor input or an actuator that fails when it gets especially hot. You're going to need to take it in since all we can really do here is idly guess. My best guess is that the car has a bad knock sensor and it cant adjust the timing enough to keep up with the lean mixture caused by high temperatures.
posted by Jon-o at 7:48 PM on July 28, 2010

If you are somewhat mechanically competent you should be able to replace the plugs and wires yourself. The hardest part is getting to the plugs themselves.

subarus have a 'boxer' engine, which means they have horizontally-opposed cylinders. great to work on, very easy to change wires and plugs. if your mechanic tells you that plugs and wires are the problem, you can do this job yourself, and save a bunch of $$$. go for it !!!

a tip: brush some anti-seize compound (available at most auto parts stores) on the threads of the new plugs, taking care not to get any on the electrode, and they'll be even easier to change next time. good luck, and hang on to that subaru !!!
posted by g.i.r. at 8:14 AM on July 29, 2010

Maybe an obvious question, but have you tried turning off the air-conditioning? Typically, you see the power-robbing effects of AC at low RPM or when accelerating, but (at least with my Legacy) on really hot days, I sometimes see performance issues at normal speeds. Turning it off for a minute usually solves the issue, but I know that's addressing the symptom not the disease.

Still, it might give you a temporary fix and point you towards the larger solution.
posted by quin at 8:23 AM on July 29, 2010

« Older what's a normal bedtime?   |   How can I make myself more competive as an... Newer »
This thread is closed to new comments.