I'm working on my daughter's 1998 Dodge Neon R/T, ~87,000mi, manual transmission, DOHC 2.0 engine. Check engine light came on with code P0172. So far I've done the following:
-replaced the spark plugs
-replaced the fuel injectors
-replaced the EGR valve
-replaced the Idle air control valve
-replaced oxygen sensor before the cat
-I've also cleaned the MAP sensor.
-I measured the fuel pump pressure (measured 49psi, spec is 50psi)
I have live data that shows long term fuel trim at idle pegged at -25.0. Short term fuel trim is also typically slightly negative at idle. The fuel trims look more normal when driving or accelerating. The Freeze frame data shows the code was tripped at idle. I'd love to send you the data but I see no way to do that on this platform. I just don't know what else to look at or do. I could replace the other O2 sensor but could that possibly help? Car runs a little rough at idle but otherwise runs fine. Any help you can provide would be greatly appreciated. Thanks
Check the fuel pressure regulator.
The fuel pressure was within spec (49psi, spec of 50psi). If the fuel pressure regulator was bad wouldn't that show up in the fuel pressure measurement?
Well I did a more extensive fuel pressure test. I got the engine up to operating temperature. I connected my scanner and verified the long term fuel trim was -25 and short term was like -1 or -2. I then checked the fuel pressure with a gauge and it was reading 50psi as it should. Therefore, I don't think the cause is a faulty fuel pressure regulator.
Another thing I've noticed about this car is it consumes a little coolant. There are no leaks or anything and it doesn't consume a lot. Not a lot of white smoke out of the tailpipe but maybe a little. It's just hard to differentiate between what I see and what is normal. Suppose the car is burning coolant. Could that cause it to run rich?
No. I would see if there is an exhaust gas leak or an ignition misfire.
Your injectors might be leaking. When they leak your dumping more fuel at idle so it runs rich. When you start driving it tends to go back to normal since the leak is no longer a factor for the most part.
That was one of the first things I did, I replaced the fuel injectors with new ones. That made no difference.
Damn. Let me think 🤔🤔🤔. Only other thing I can really think of after reading everything else . Your cat is bad or exhaust leak and o2 is throwing ghost codes. Head gasket is another possibility. Oh actually no you don't always hear or smell an exhaust leak. I had a car come to me that had o2 codes. Turn out the exhaust manifold gasket was bad. When I removed it half the gasket was gone. There was no smell or sound. It was found when I put my hand around the manifold and felt a little air blowing.
I'm not sure how the cat could cause the problem. I thought that the O2 sensor used for fuel trim calculation was before the cat. An exhaust leak is an interesting idea but since the exhaust manifold is on the rear of the engine and covered it's hard to see or examine. The engine is actually pretty clean for its age. I did have the head gasket replaced at about 60,000mi. Maybe it's going bad again, I don't know. But since it is consuming a little coolant that seems like a distinct possibility. The fuel injectors are Herko brand. They are new. Walmart carries this brand so they can't be that bad, right? Buying Mopar fuel injectors is tough. I go to the dealer and they don't go back to 1998 for parts. I bought some refurbished ones on EBay only to find out they didn't fit when I tried to install them even though Ebay said they fit my car.
Autozone carries Sorensen brand. Are those any good? I don't know. Anyway, thanks for the ideas.
Your response has brought to mind a question, how could a head gasket issue cause an engine to run rich? I don't understand the mechanism here so if you could enlighten me I'd appreciate it. Maybe I should try some of that head gasket sealer Scotty talks about. Would that have any chance of solving this issue?
Too keep it short and simple as possible. When you have a exhaust leak near or around the o2 sensors you either loses gases and or such air in. When that happens the o2 sensor will pick it up. It sends that data to computer. The computer will then readjust trims based on that info. In turn you can get rich/lean conditions . Even tho the car was never running lean or rich. I hear a lot of people saying the downstream only monitors and upstream had nothing to do with trims but that is 100%false. Both are very important to having proper air fuel ratio. I posted something on here awhile back about a car I fixed. It had something like 4 or 5 codes plus a rich codes and was running terrible. I found out the downstream o2 was bad and caused everything to happen. Too top it off there was no code for that sensor either.
I've got a new downstream O2 sensor for that car in my garage. Maybe I should just replace that and see what happens. That might be the easiest thing to do and if it fixed a car for you it might fix mine as well.
Yeah I've seen o2 sensors cause a ton of issues. In My Mercedes I had a bad downstream o2. It threw codes for the map, maf, upstream O2 and misfire code on 3 cylinders. It only had a slight idle problem, nothing else. Around 2 years ago I read a article in a engineering magazine it stated that undiagnosed o2 sensors are responsible for up to 25% of misdiagnosed or undiagnosed vehicles. I found that really interesting. It doesn't hurt to replace if you have it already. Question do you know how to read live data? Do you have a scanner that lets you graph o2 sensors voltage? Also do you have a small propane tank or map gas? There is a little test you can do that might help figure out if the o2 or cat is to blame. I'm going to reach out to a few mechanics I know and see if they have any ideas. We will figure this out, we just have to keep going down the list of possible issues.
I just thought of something. Check your timing, your timing belt might be stretched and caused it to jump a tooth or two or your tensioner is no longer working properly and it's causing it to jump a tooth or two as well.
I have a couple of scan tools both read live data. That's how I know the computer thinks the car is running rich at idle. The car has also thrown P0172 and the freeze frame shows it occurred at idle. I do have a propane torch. Regarding timing, when I look at the live data at idle the timing advance does jump around, from, say, 5-17 degrees. I know that's not good but I'm not sure how that could cause this problem. The car runs too well at speed for the timing belt to have jumped a tooth. The only time it runs roughly is at idle.
Your timing belt is no good it shouldn't jump around that much . It probably heavily worn. Im not sure how to explain the whole timing thing. I'll give a basic answer. If your timing is Advanced you will run lean. If the timing is retarded you run rich. So if your timing is off your ignition cycle will be early or late. You can run lean or rich depending how much it's off by. As for the propane remove air box start car and introduce gas to air steam. When you do this your car will full rich in a second or 2. It shouldnt take any longer then that When you remove the propane it will go back to normal. You want to watch the live data for both o2 on a graph. When you remove the propane there should be a slight delay followed by a rapid but steady drop to normal. If it takes more then a second or 2 for it to respond you know your sensor is bad. If it responds with a slight delay but takes awhile to drop to normal the cat is bad.
I replaced the O2 sensor after the cat and the car exhibits the same problem. I seems even worse now in that it dies frequently when idling. It still runs fine when driving and accelerating and I think the coolant leak is coming from the thermostat housing but it's still running quite rich while idling. I have no idea why. I'm thinking I'll just take it to the local mechanic.
Unfortunately that cost alot of money. If you are unsure you can trust them check the head gasket your self. Harbour freight sells a tester that is like 20 Dollars and you can check for yourself.
https://www.harborfreight.com/combustion-leak-detector-64814.html
Here is the link for the test kit.
Thanks. Not too long ago I did a head gasket test and there was no combustion exhaust in the coolant. But I do think the coolant is leaking to the outside through the head gasket. So still a head gasket problem but not the kind this test will find. So I've decided not to spend the $4000 to $5000 thousand these guys want to do the job. Besides, they aren't sure if they can get the parts they need. I'm going to try some head gasket sealer and live with the problem at least for a while.
Sometimes it's hard to diagnose over a message. You might have to bring it to a mechanic like you said. A lot times when a part is replaced and makes it worse it's because the part is aftermarket and isn't working properly or the new part is defective. I didn't know you had a coolant leak. I didn't see that in the original message. I totally forget one thing I just remembered when you said coolant. Check the engine coolant temp sensor. When they fail they can mess with fuel trims. You should double check or check the iat coolant temp sensor, maf and map. Another thing is can be that I just remembered as well is a clogged or pinched vacuum line . There is so many possibilities it can be I really wish I had the car in front of me. I could be alot more helpful. After rest everything again. It really seems like a injector issue, fuel regulator or a clogged or pinched vacuum line.
So I took it to a mechanic. They are saying it has valve issues and needs a new head. They are also saying the coolant leak is coming from the head. The cost will be in the thousands to fix this if I choose to. It's hard to believe because it runs so well at speed. I only notice a problem at idle. But there is a coolant leak and if it is coming from the head then it at least needs a new head gasket which will not be cheap. So now I guess I need to decide what to do. Not sure it's worth fixing. Is it worth getting a second opinion?
I used that stuff on my Saturn ion and it helped. I got another 35,000 miles till it just went out. 4,000 to 5,000 grand is a complete rip off. They are trying to take you to the cleaners. I pulled up some info on it. Parts and labor cost around 675 to 725. If the engine is warped it will be another 200 to 300 to have it mechined. 875 to 1025 is what it would cost depending on few things. For 1300 to 1500 you can have a used engine with around 135,000 installed. But let's be realistic the car is old and either job will exceed it's book value. Personally I would just continue driving it till it goes and find something else in the meantime.
You're right on. I really don't trust the guys that much. I think they just didn't want to do the job. It was a Christian Brothers. Not sure chains get the best mechanics. Anyway, I believe there is a coolant leak at the head gasket. But how could the valves be bad when it runs great with plenty of pep and smoothness. Only idle is an issue. Furthermore, the day before I brought it in it wouldn't idle at all. But the day I brought it in it was idling although a little roughly. I've never heard of a valve issue fixing itself from one day to the next. This intermittency does not suggest a valve issue.
You can have carbon build up on the valves preventing it from opening and closing all the way. You can have some mild systems. If valves are actually damaged they won't magically fix themselves. They are definitely trying to scam you. It's unfortunate but it does happen. Worst of all these people almost always target women, especially young woman. If it's leaking from that area then your gasket is failing. Try the head gasket repair like you said. You have nothing to lose at this point. I have heard some good things about bars and blue devil . Just note if it does work it might last a couple of days or over a year. There is no time line on how long Your most likely going to have to remove the thermostat to use that stuff. Most of them call for that .
I put some K-Seal in the system today. Not sure if it worked yet since the leak was pretty minor. Will check coolant consumption in a few days to know for sure.
Let's hope it's works . If it doesn't and you use another product. Make sure to remove thermostat. It can clog system and prevent flow making products ineffective. I' will reach out to a mechanic I know who is a engine specialist. I know he has used this stuff a number of times. I'll ask what he recommends you use in your Case.
Thanks, the instructions didn't say to remove the thermostat so I didn't. Probably would have been a good idea though