420A Rough idle when cold, hard start and fluctuating idle when warm.

Nova95 · Jul 7, 2022

Hey guys. I just picked up a 1996 Mitsubishi Eclipse RS with 180,000 miles that I'm having issues with and have finally decided to come here for help.

Originally the car was throwing a code for a bad 02 sensor and a P0300, random misfire or multiple misfire. The only real issues are when you start her first thing in the morning her idle fluctuates crazy and it kind of sounds like it's just going to die. Eventually the idle smooths out and sounds perfect when she first hits normal operating temps. Shortly after her idle starts to fluctuate up and down quite a bit, but it's a consistent up and down, it's not random.

Once driving though she feels fine. No stuttering or bogging, she feels completely normal. When first coming to a stop she sounds fine too. But after a few minutes at a red light her consistent up and down idle comes back.

I've since replaced the 02 and now only have the random misfire code. We tested each spark plug (car on, pull a plug) and everything checked out fine but to be sure we bought new and threw them in anyways. (Previous owner for some reason only changed the plugs for the first 3 cylinders). I also noticed cylinder 3s plug smelled of fuel and the rest did not (That I noticed). We chased a few potential vacuum leaks and no go. I noticed they replaced the coil, ran a new vacuum hose and wired in a new camshaft sensor so I'm thinking the original owners were also trying to chase this issue and just gave up.

The car is sounding worse at idle now and when warm she is really hard to get started and today I wasn't sure she was even going to start. Her pump kicks on strong each time and while turning her over I did smell gas so I think she's getting fuel just fine.

With her not throwing any other code, where else should I look for issues?

luv2rallye · Jul 8, 2022

Idle going up/down is usually an air leak in the intake system somewhere. Possibly throttle body (TB) shaft seals, leaky throttle body or intake manifold gaskets, intake hose clamp leak, vacuum hose leak, etc. Could also be bad IAC, TB itself, Biss screw misadjusted. Spray TB, gaskets, and intake hoses and connections with soapy water while engine running and look for bubbles. At suspect input areas you can also verify by spraying WD40 to see if engine speed drops. Finally do leak down test.

CKOPOCT · Jul 8, 2022

Second what luv2rallye says. If your problem seems to be only at idle, predominantly when the engine is cold, then there is either not enough fuel or too much air. The hunting and surging is due to the ECM trying to compensate. At 180K and 26 years old without a major system rebuild an air leak is very possible... and relatively easy to check for.

If your air leak and leakdown tests don't produce results then I would check for fuel delivery issues. Injectors, fuel filter, coolant temp sensor, etc. I think your problem will be air though...

Marley 1g dsm · Jul 8, 2022

Check air start after mass air flow, idle adjustment screw. Note possibly the #3 injector leak you stated something about spark plug and you smell fuel.

BLACK'98DSM · Jul 8, 2022

Based on the symptoms it could possibly be a throttle position sensor (TPS). But usually it would throw a code if the readings were jacked up.

Nova95 · Jul 9, 2022

Okay so today after work we used carb cleaner to test all of the vacuum lines. Everything is fine except for the Egr valve pipe/tube I'm guessing it's called? It's an aluminum looking pipe sitting on the back side of the intake manifold with two bolts and right below it is the IAC valve. The gasket there is torn and you can hear and feel the air being sucked in. I'm picking up a new gasket tomorrow but I'm wondering if that alone would be the culprit of all of my issues or if the Egr valve assembly is also bad? I'm still only getting the misfire code but today after work she wouldn't start at all. Once she finally did an hour later, she blew white smoke out the back. I assumed maybe a bad head gasket, but it didn't smell like coolant, only gas. We also noticed today that the exhaust manifold is cracked in two places so I'm guessing that isn't helping any either.

BLACK'98DSM · Jul 10, 2022

Vacuum leaks can cause a lot of problems if they're big leaks. Start by getting that leak fixed and then see what happens, skip the guesswork for now.

And for the exhaust manifold. You can just weld the cracks closed. For much cheaper than buying another one. I've done it a few times just need to make sure that iron is compltely clean so the welds take smoothly. Doing this with the manifold off the car is preferable.

Nova95 · Jul 11, 2022

Okay fixed the leak from the Egr, I no longer hear the hissing noise of air, idle seems better, and there's no code yet though I don't think I've driven it enough to throw the code again if it was to. But she's almost impossible to start now. She seems to have a "sweet spot" in the gas pedal where if I hold it just right she tries to sputter to life. She's easier to start with the radiator cap loosened, then once she starts I have to tighten it back on. Then about 15 minutes of driving her temp guage jumps up pretty high, then comes down. She pretty much does this the rest of the ride, never staying up hot. Having the AC on doesn't seem to make any difference. She makes a whining/chirping noise that I think is coming from the radiator fluid leaking out of the reservoir and onto the belt but could be the water pump also. I don't have a leak down kit yet, I'll get one friday but for now I'll have to settle for a compression test when I get home. I'm thinking I'm looking at a sticking thermostat, bad water pump, or failed head gasket? Is replacing the head gasket in this car with the engine in the bay relatively easy? I've done head gaskets

CKOPOCT · Jul 11, 2022

No need to remove the engine to do a head gasket. Is your coolant reservoir over full or is it cracked and leaking?

As to "her temp guage jumps up pretty high," don't know what this means. "Pretty high" is pretty vague. If the gauge hits the red it could be trapped air, a gauge that isn't as accurate as it once was, an air flow problem, coolant temp sensor, etc. If you aren't overheating and if the gauge isn't climbing and staying in the danger zone then I doubt it's the head gasket.

I would check the coolant temp sensor. Easy to do and won't cost you anything. A bad CTS will make it hard to start and can cause running issues.

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Nova95 · Jul 13, 2022

CKOPOCT said:
No need to remove the engine to do a head gasket. Is your coolant reservoir over full or is it cracked and leaking?

As to "her temp guage jumps up pretty high," don't know what this means. "Pretty high" is pretty vague. If the gauge hits the red it could be trapped air, a gauge that isn't as accurate as it once was, an air flow problem, coolant temp sensor, etc. If you aren't overheating and if the gauge isn't climbing and staying in the danger zone then I doubt it's the head gasket.

I would check the coolant temp sensor. Easy to do and won't cost you anything. A bad CTS will make it hard to start and can cause running issues.

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Originally the reservoir had a crack that it was leaking out so we fixed that.

CKOPOCT said:
No need to remove the engine to do a head gasket. Is your coolant reservoir over full or is it cracked and leaking?

As to "her temp guage jumps up pretty high," don't know what this means. "Pretty high" is pretty vague. If the gauge hits the red it could be trapped air, a gauge that isn't as accurate as it once was, an air flow problem, coolant temp sensor, etc. If you aren't overheating and if the gauge isn't climbing and staying in the danger zone then I doubt it's the head gasket.

I would check the coolant temp sensor. Easy to do and won't cost you anything. A bad CTS will make it hard to start and can cause running issues.

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Sorry, what I mean by "jumps pretty high" is that the car sits at the halfway at normal temps But when she starts to act up, she goes up just before the H. So pretty much she was running hotter than she should, but not entirely overheating. Either way though I just found that the water pump is in fact toast, she's now spitting fluids onto the pulley, all down the bottom side of the car, and spitting bubbles into the raidator. Car is parked now and I'll do a water pump, head gasket, potentially a used head offline (resurfaced) and timing belt since there's no point in going as far as the water pump and not changing the rest. Thanks for all the help folks hope after this I'll get to actually enjoy my dream car.

waltah · Jul 14, 2022

Just measure the resistance of the CTS in place with the car at ambient. Look at the table posted; if it's close to what it should be at that temp you are good to go. What causes serious trouble is a shorted or open sensor -- most commonly a wiring issue due to brittle insulation from the heat.

The compression test is the story. If it's okay there's no need for a head gasket job.

A really shot water pump not only doesn't provide enough cooling but it allows air to enter the system and air in the coolant will cause all the other problems.

I wouldn't go beyond a water pump and compression test without better evidence than you've got.

Nova95 · Jul 14, 2022

Couldn't get the car started today (killed the battery trying) so ran a compression test at cold engine since it's the best I could do. Cylinder 1: 110, cylinder 2: 105, cylinder 3: 100, cylinder 4 108. Numbers seemed fine enough so replaced the coolant temp sensor, realized the water pump requires tinkering with the timing belt so held off till I could buy a new timing belt to go with it. Came back out an hour later, Got the car started (on my backup battery) after loosening the radiator cap. Drove to the liquor store and back fine (15 miles). Checked the oil cap and dipstick for shits and giggles after I saw steam and bubbles in the radiator overflow and noticed my oil looks like coffee. Bleh...

CKOPOCT · Jul 15, 2022

100 is the low service limit for compression. Your numbers should be up above 150 (ideally 180-220). With what you are describing sounds like a head gasket. At 180k miles there could be more going on as well (rings, valve guides, etc.). A leakdown test will likely show a bad head gasket but could also reveal other weaknesses depending on how bad the hg is.

I'd do a leakdown test to see what it reveals. You could also put a little oil into each cylinder and redo your compression test. That could help id ring issues...

Nova95 · Dec 9, 2022

So hopefully it's okay to post back here instead of making a new thread but if not just let me know. I have since fixed the head gasket and she now runs better than she ever did, has a consistent idle, and doesn't overheat. Now she's doing something weird. Once she gets to operating temperature, at 3000rpms the needles spikes up and down like crazy, and she sputters/hesitates. If I let go of the gas and come off of the 3000rpms it stops. Same if I had it and get above the 3000. If she stays doing the sputtering at the 3000rpms and I don't do anything to change the rpms, she eventually will just shut off driving down the road and refuses to start until it sits about 10-15 minutes. She still is throwing the po300 code but I feel like I have checked everything. No vacuum leaks, new spark plugs (gap triple checked at .5) new wires, new coil, cam and crank wiring harness show their respective voltages.He compression is sitting at like 105-110 between all 4 cylinders. I know that's on the low end, but would that really cause the weird rpm/shut off I'm dealing with? I'm about to buy new crank and cam sensors to try but thought id check for input first

BLACK'98DSM · Dec 9, 2022

That's definitely low compression. I'm pretty sure the FSM lists 130 psi as the service life. Freshly built 420A makes well over 200 psi.

But, I don't think that is the cause of the issue you described. That sounds to me like a loose ground connection or a bad camshaft position sensor. Whenever my CPS went bad the rpm needle was jumping up and down.

Nova95 · Dec 9, 2022

Nova95 said:
So hopefully it's okay to post back here instead of making a new thread but if not just let me know. I have since fixed the head gasket and she now runs better than she ever did, has a consistent idle, and doesn't overheat. Now she's doing something weird. Once she gets to operating temperature, at 3000rpms the needles spikes up and down like crazy, and she sputters/hesitates. If I let go of the gas and come off of the 3000rpms it stops. Same if I had it and get above the 3000. If she stays doing the sputtering at the 3000rpms and I don't do anything to change the rpms, she eventually will just shut off driving down the road and refuses to start until it sits about 10-15 minutes. She still is throwing the po300 code but I feel like I have checked everything. No vacuum leaks, new spark plugs (gap triple checked at .5) new wires, new coil, cam and crank wiring harness show their respective voltages.He compression is sitting at like 105-110 between all 4 cylinders. I know that's on the low end, but would that really cause the weird rpm/shut off I'm dealing with? I'm about to buy new crank and cam sensors to try but thought id check for input first

Do you happen to live in Missouri? Lol.

BLACK'98DSM said:
That's definitely low compression. I'm pretty sure the FSM lists 130 psi as the service life. Freshly built 420A makes well over 200 psi.

But, I don't think that is the cause of the issue you described. That sounds to me like a loose ground connection or a bad camshaft position sensor. Whenever my CPS went bad the rpm needle was jumping up and down.

You don't happen to live in Missouri do you? Lol. Are you talking about the ground from the engine to the frame being loose? Because if so I checked that she's good. When your CPS went bad, did your needle jump crazy at specific rpms? Previous owner replaced the camshaft sensor clearly so I'm looking at replacing that with oem as my next step at this

BLACK'98DSM · Dec 9, 2022

Nova95 said:
You don't happen to live in Missouri do you? Lol.

Unfortunately not. We're about 500 miles away haha. If it weren't for that I would definitely come lend a hand.

Nova95 said:
Are you talking about the ground from the engine to the frame being loose? Because if so I checked that she's good.

There are multiple grounds in the engine bay. One underneath a belhousing bolt, one between IM and firewall, one from fuse box to body, one from battery to strut tower, and each relay that is vertically mounted on or near the firewall is grounded from their mounting point. Obviously not all of them would cause your issue but they all share the same zinc coating which can become compromised and they will corrode, eventually causing loss of connection. Either that or the bolt could have been left loose previously.

Nova95 said:
When your CPS went bad, did your needle jump crazy at specific rpms? Previous owner replaced the camshaft sensor clearly so I'm looking at replacing that with oem as my next step at this

It's been a little while since mine went bad but I remember casually driving and seeing the tachometer intermittently bouncing above and below the actual engine speed. It felt like the engine was misfiring as well. It is possible that the previous owner installed a cheaper one. OEM is definitely the way to go with those but something like Standard Motor Products would suffice. Just know that most if not all of the ones sold on Ebay are no good.

Nova95 · Dec 11, 2022

Well a Duralast Cam Sensor didn't fix it. The crankshaft sensor won't be here until Tuesday so won't have a update on that till then. Got to work and the check engine light started flashing and now she's throwing p1391 instead of the po300 she has been throwing this whole time.. Maybe the cam sensor did at least help, she also didn't cut off mid drive so. This has been the most strange vehicle issues I've seen.

Nova95 · Dec 14, 2022

Update neither the camshaft or crankshaft position sensor fixed my problem. Im starting to think somewhere in the wiring I may have a failing wire that is causing intermittent issues, or a bad ECU throwing my po300. Going to check all of the wiring throughout the engine bay next decent day I get.

Nova95 · Dec 18, 2022

All of the wiring looks good. I have noticed that after the crank and cam sensors, she no longer flickers and bogs/stutters, she drives and idles perfect, but she flashes consistently at 3000 rpms. It's more of a steady/consistent flash now. But still the only code she throws is the po300. Is there a chance that the cat converter is clogged enough to throw a multiple cylinder misfire, but not be clogged to the point that the car runs horribly?

BLACK'98DSM · Dec 22, 2022

Has the RPM needle stopped bouncing? That's one of the reasons I called out the camshaft positions sensor. Good to know you replaced it but I'm curious if it solved that problem.

As for your random misfire code. That might be related to the low compression. With enough blowby it could be hinting at a misfire. It's actually quite common for cars with low compression to throw misfire codes. At some point you'll probably need new pistons and rings.

Nova95 · Dec 24, 2022

BLACK'98DSM said:
Has the RPM needle stopped bouncing? That's one of the reasons I called out the camshaft positions sensor. Good to know you replaced it but I'm curious if it solved that problem.

As for your random misfire code. That might be related to the low compression. With enough blowby it could be hinting at a misfire. It's actually quite common for cars with low compression to throw misfire codes. At some point you'll probably need new pistons and rings.

Yeah after replacing the camshaft sensor she only did it like 5 times per trip, not the whole time like she used too. After putting the crankshaft sensor in the needle doesn't bounce at all anymore. So it seems like she's as good as she's going to get until I can afford to rebuild the engine completely to fix the low compression. Would using thicker oil get me better compression in the meantime? Or maybe going back to conventional instead of synthetic?

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