420A Rough idle/no power. Stumped.

[aaronth777]

Hello, this is my first post on here. I have a 1999 eclipse gs that's basically stock (only aftermarket cold air intake, exhaust system, and bored throttle body), and it has a weird problem. Yes I've done research and used the search bar for two weeks now, but I'm stumped and need help.

I've had this car for over two years, and whenever it was raining or really damp in the air (like heavy morning dew) it didn't like it. It would run fine for about 5-10 minutes, but then it would have sudden losses of power. Like I had the pedal to the floor, then it would randomly sputter and I wouldn't even be able to keep my speed. I could restart the car while I was moving (perks of being a manual ) and it would have power again for like 10 seconds, then do the same thing. Then when I got out of the heavy dew and into the city, it would quit throwing a tantrum and run perfectly fine. My guess was that water was getting in the car somewhere somehow, causing issues, because if I started the car after it rained, it would idle rough with a huge lope, having no power for like 1-2 minutes, then ran fine again, almost like it had to get the water out of its system. I ended up replacing every gasket in the head, along with a few sensors, and the problem went away for a few months.

I thought I had fixed it, but then a few days ago, it started doing it all the time whether it was damp or dry outside. When the car is completely cold, I will start it and it will run fine for 5-10 min. Then it starts getting a really rough idle, with a massive lope, and when I try to accelerate it bogs down heavily. Restarting it doesn't do any good, and only goes away if the engine gets cold again. I'm guessing this could be related to the issue I had before, but I'm not sure.

When it was idling rough, I unplugged wires one by one, and every cylinder works, because the idle noticeably changed after I unplugged and re plugged every wire. Other forums have suggested injectors, fuel filters, or timing belt a tooth off, but that can't be the issue because it runs perfectly fine in those 5-10 min on a cold start. Every vacuum line is sealed (as far as I know, I checked most of them, but I guess I could have missed one), and I've even reset the ecu by unplugging the negative battery terminal for about 3 hours, but it didn't change anything. I should mention that the car only has a downstream O2 sensor, not an upstream one, because the aftermarket headers are poorly designed and push the O2 sensor into the radiator fan cover, breaking it after about a month every time. But the car has been like that for 2 years and ran fine for months like that, so I didn't think that could be the cause. I doubt this is related, but there is a decent oil leak somewhere, and I'm not sure where it's dripping out from, but that's another issue for later (I always make sure it's good on oil before starting). Idk if it's throwing a cel other than the missing O2 sensor, I can't get it anywhere to check it.

My only guess would be something's bad in the ecu? But apparently 1999 420a eclipse's never have an issue with the ecu. A friend suggested a faulty egr valve, but I have no idea. Sorry for the long post, but any help will be GREATLY appreciated!!

 

[ThunderChild]

Not having a front O2 is a big issue. That's what's adjusting your fuel trims. You need to address that asap.

Past that, sounds like possibly a bad MAP or MDP (as some like to call it) sensor, or a bad vacuum leak.

The map sensor is on the passenger side of the intake manifold, and is prone to carbon build up and moisture with age.
Depending on the oil leak, I've had previous cases of the CKP (crank) sensor on 420a's getting oil inside them and cutting spark out.

Mostly you need to address the O2 issue, have a shop weld a new bung in a better position and get a working sensor in there.

 

[aaronth777]

Thanks for the fast reply! I know I need to get the front O2 sensor fixed, just never got around to doing it. That's the next project after I fix the current issue.

I had thought the MAP could be bad, but wasn't sure about it. I have a friend with the same car as mine, so I'm gonna try his MAP on my car to see if that fixes anything. The crank sensor is a really good idea though, I hadn't thought of that! I'm almost 100% sure the oil is leaking from the back of the engine somewhere, and could have gotten into the crank position sensor. As long as the weather cooperates, I'll look at this stuff tomorrow and post what I find.

 

[dsmcrzy]

Stop trying to throw stuff at it. Get the front O2 sensor hooked up and that will most likely solve your issue. Like tametalon said that sensor is what the ecu uses to adjust fuel trim. With out it it will run like crap just like you are describing.

Start with the obvious missing sensors before you try swapping out the rest.

The rear O2 sensor has ZERO to do with how the car runs. It's just an emissions check to make sure you have a cat.

 

[Wrathofwalrus]

Remove the Fan that's in the way and get that o2 in there. No front o2 sensor can cause a no start, Driving without one I wouldn't even attempt and explains your no power symptoms . It Doesn't know what to do without the front o2 so it just dumps as much fuel as possible.

 

[Wrathofwalrus]

The front 02 sensor is most likely your current issue. Don't just play the guessing game and throw new parts and sensors at it. Get the car scanned if you don't have an obd2 scanner.

 

[X2M7NOZ]

Assuming the 420a front o2/ecu operates the same as the 4g63 it is not your issue. The ecu only uses the front o2 for idle and cruising, that is called "closed loop". Closed loop means the ecu is checking it self according to the previous engine cycle and predicts the fuel and spark. If the ecu does not get a signal response for x time it forces itself into open loop. Open loop literally just uses the current air flow sensors to calculate fuel and spark. Open loop is used if the ecu see a voltage reading higher then x, so literally if your throttle is not steady the ecu will use open loop, which means your ecu regularly ignores your o2 sensor. I still highly recommend getting that taken care of still because thats a huge rob in mpg when cruising. My front 02 has been a wide band for about two years and is not ran into the ecu... car ran fine before i tore it apart . Back to your issue though, It sounds like a choke issue, possibly tps sensor. After that 5-10 minutes the car should be fully warmed and using more sensors then when first started/warming up. The thermostat send a reading to the eco for fuel trims on 4g63 so consider that as well.