Stumbling/Mis-fire under load at low RPM...
 

I see a couple of similar threads and I'm gonna monitor both but since I don't have a CEL and I have one symptom not mentioned in either I'm gonna start a new thread just in case it's a different issue...


The car is my son's 1995 Saturn SL1 SOHC AT with 120,000 miles. I have done quite a few things to it over the past couple of years to keep it in good repair and tune and it has been reliable and economical. Only issue has been that it uses about 1 qt/1k miles. The plugs and wires were replaced about 12k-15k ago.

Recently it has begun to stumble hard under low RPM load. Level driving or acceleration where the RPM is high and the gearing is low seems fine -- no missing. However, when it shifts into high gear and the RPM drops or if you start up a hill with it already in high gear, it will stumble in hard jolts. The symptom that I haven't heard others describe is that after it stumbles a few times there is a smell of raw gas in the cabin that goes away after it stops missing. There is no steady or flickering CEL.

Maybe interesting is that though the miss/stumble is hard -- as I said, feels like a little jolt in the whole car -- the engine sounds smooth and I don't see any "ticking" of the tachometer. Of course that could be, and probably is, a red herring that results from the fact that the road noise in that little car is so loud that it's really hard to be that discerning about the various noises and the tachometer is probably not that sensitive to small, momentary, changes.

I've never replaced the fuel filter -- doing so looks like a pretty tough proposition -- and don't know when it was last changed. But this seems/sounds to me more like an ignition problem than a fuel supply/flow problem.

I just swapped vehicles with him today. I confirmed the above description of the symptoms but I won't actually get a chance to start working on it before Monday morning.

Any ideas to focus my initial troubleshooting?

The first thing I'd do is check the plugs. They can tell you a lot about the condition of an engine. Just use your trusty Haynes manual to see what the color or condition indicates. There's a handy guide inside the back cover. After that, I'd clean the throttle body and change that fuel filter.

I just changed the plugs. The old ones didn't look bad but they had burned down to the point that the gap was nearer to 50 than it was to 40.


As for the fuel filter, while I was at Oreilly's I picked up a new WIX filter. Not what I expected in form factor or price. Still, it is what it is. The real surprise was when I looked a little closer at changing it out. How the heck are you supposed to get to this thing? Is there a little secret trick that makes it easier than it looks?


I figure I'll pull the fuel pump fuse and then run the engine until it runs out of gas to minimize the fuel and pressure in the line but after that I'm not sure where to start. If I pull the wheel and fender liner will I gain reasonable access? Is there a better way?


Thanks in advance.

I'm not sure how the hell to get at it, Howard. I went out and looked at mine and it looks like a real PITA to get to. If you change yours, please let me know how you go about it. Mine's running fine, but it looks like the original and I have no idea what the real mileage is on this car.

Ok, like a lot of things it looked harder than it actually was. I was able to replace it from the top without removing anything other than the throttle body air inlet. I didn't even remove the old clamp that holds it to the frame. Leaning in from the driver's side I was able to turn my right arm counter-clockwise and reach back to the hose coupler on the bottom of the filter. After a couple of minutes of wishing I had a longer thumb and forefinger nails I was able to get the bottom quick release off. Then it was just a matter of releasing the top coupler and sliding the old filter out. Putting the new one in was a snap -- pun intended.


Btw, I put a plastic tarp under the car to catch the gas that came out when I disconnected the old one and it caught probably not more than 1/8 cup or less. What had pooled on the tarp had a lot of rust particulate matter in it.


I'll take it out after a while and see if the plugs and/or fuel filter solved the cutting out problem and will post back.

Well, the results put me in a nether world. I'm not sure how to interpret them other than to say the plugs and filter didn't "completely" eliminate the symptoms -- just reduced them. Reduced them enough that for a while I thought the problem was fixed.


It still cuts out when under load at low RMPs (in high gear) but has to be under more load (behind the power curve) than it did before I replaced the plugs and filter. I also noticed that when the RPMs are up above 2000 in the lower gears [and not accelerating] that the engine sounds/feels like it is varying -- dropping off and then picking back up -- a bit. That may not be new -- I may not have been listening for it before.


If someone has a more directed approach to figuring this out and resolving it just let me know and I'm happy to give it a shot. Otherwise I'll clean the throttle body -- though it looked pretty good when I had the air inlet off -- and will pull and clean the EGR valve and see if that makes any difference.


Grrr...

Take a good look at the connector on your ECTS, Howard. Mine was cracked and I didn't realize it until I'd already bought a new ECTS and pigtail. Probably could have just gotten away with replacing the pigtail.

I will Charlie. I replaced it shortly after my son bought the car but will look at it again.


Does this cause the symptom -- stumbling/cutting out when behind the power curve in high gear?

Yep, that could be one of the things. That damn ECTS seems to effect a LOT of things. I never knew that engine temp could be so critical to the computer cars. Back when I first started farting around with cars, they had point style distributors and carburetors. Sometimes I long for those days again!

The ECTS looks good. It is the brass type and I put a new connector [and pigtail] on it back when he first got the car. It is making good connection and there are no leaks. Beyond that I have no idea whether it is working correctly or not.

Does this car throw out codes that don't turn on the CEL? I've been assuming that it isn't self-detecting any problems because it isn't turning the CEL on. That may not be the case. My local Oreilly Auto Parts doesn't have a scan tool that reads OBD I. I'll call around tomorrow and see if any of the other auto places do. I've read that you can read them manually with a paper clip. If I can't find a scan tool I'll give that a try. Sure would be nice if the car's diagnostic system could help a little bit here.

paper clip

https://www.saturnforum.com/forum/sh...ighlight=codes

I have to use a paper clip on mine, Howard. It's a very simple procedure. Back in the 90's when I worked for Autozone, they used to give away a "key" for the GM cars that plugged into the diagnostic link. I wish I still had one, but the paper clip works just as well. The two holes you need to put the paper clip into are the two on the top right side of the connector link.

Thanks Derf and Charley. After posting last night I remembered the paper clip method and used it. It only throws out the initial code 12.

The battery was disconnected for a while so any warnings would have been erased but if there had been any error states they would have been stored.

I would have thought that a hard misfire would have thrown off some sort of code...

http://www.differentracing.com/tech_...diag_info.html

…and…

http://www.differentracing.com/tech_...dtc_chart.html



I think it's interesting, though I don't know what to make of it, that the cutting out is occurring where, in the past, pinging from low octane fuel or too advanced timing would be likely.

Btw, I measure the resistance of the ignition coil packs. One read 7.4K across the towers and the other read 7.48K. All 4 towers read open to ground.


The tests I've seen described in different threads say they should read 8K between towers. I don't know if 420-500 ohms (5-6%) variation on an 8K scale is meaningful for these devices or not.

The first thing I did back at the beginning of this troubleshooting process was to remove the plugs and inspect them and the plug wires. The plugs looked fine but had burned enough of the tip down that the gap was too wide so rather than re-gapping them I just bought new.


The wires all measured between 3.5k and 7.5k, increasing by their length, and I always assume that anything under 12k is ok.


This morning when I was checking the resistance of the ignition coils I don't know why but I decided to recheck the plug wires. Two of them read 4.4k and 7.4k and the other two read open. That was a surprise to say the least. After wiggling the meter probes around for a while to make sure that I was making good contact and seeing the wires start to show continuity and then opening back up I decided that the graphite core must be broken down. I replaced them with a new set of BW pre-cuts and wa la, the little dickens is running great again and not a hint of a misfire.


Since the wires read good the first time around I'm not sure how I could have shortened this process any but it is a little irritating that I took so much of my, and others', time on what was one of the initial/typical suspects for this kind of symptom.


At any rate, thanks all -- the issue is resolved.


When the weather warms up a bit I'm gonna pull the head and pan and give it a good cleaning. The oil is getting dark and thick way to quick after an oil change. I think it being gunked up so bad, along with the possibility of some leaky seals, is why it is using oil and seems to have excessive blow-by.


Happy New Year all.

Since your car is pre OBDII, this is going to be a little harder. I don't think it is a fuel problem, but it is possible. Usually a fuel problem occurs at high RPM under full load, that is wide open throttle as you approach redline. I do not see any evidence that you have tried this so I don't know. If you have done a full throttle test in the lower gears and it did not cut out as it approached redline, then I believe your fuel delivery is good.

It could still be a fuel problem if one of the injectors is clogged.

My first question is are you sure its the engine and not the transmission? Have you serviced the transmission on schedule? Have you checked the condition of the ATF? It should be bright red.

If you are sure it is the engine, then the conditions you describe sound a lot like a burned valve, so I'd suggest a compression test or a leak-down test. A leak-down test would be better for finding a burned valve.

The next thing I'd look at is the spark plugs and the spark plug wires but since you just changed them, I'd be more inclined to check the wires. The OEM wires are very robust and can easily last the lifetime of the vehicle, I never change them. Most aftermarket sets do not meet the resistance requirements of the coil and are not nearly so robust. Be especially careful of using "High Performance" ignition wires, they are not compatible with a factory ignition system. Their low resistance changes the characteristics of the spark and not in a good way, always resulting in LESS energy to the spark plug tip.

I do prefer the platinum tipped or iridium tipped plugs. Their smaller cathode (tip) results in a concentration of and a stronger corona to form resulting in a hotter spark that can ignite a larger range of fuel air ratios that might result from a partially clogged injector.

If you have regular plugs, I would not advise changing them right now, you have another problem and it is better to solve it than to try to cover it up. You can use a spark tester that clamps or is held adjacent to the spark plug wires to check for spark. I use an old inductive timing light for that since I have one laying around and its not good for anything else on these new cars.

Other possibilities are a bad sensor wire from the EI module so the engine is not timed as best as it could be, a jumped timing chain but that is very doubtful with this engine, a cracked coil on the EI module which is very likely and would probably be visible at night with the engine running, corrosion on the coil terminals and or carbon tracking down the side of one of the coil terminals again usually visible at night.

The symptoms you list are usually due to a low energy spark or low compression, not often due to fuel issues.

Glad to see it got resolved, while writing this post, I had to run to the store so I was gone about an hour, finished up when I got back and now see that you have posted the resolution while I was gone.

If you still have the OEM wires laying around, I would keep hold of them, just in case.

Thanks Keith. I appreciate the response.


Happy New Year.

Good work, Howard. It's amazing how such a simple thing as bad wires can cause such problems. I'd try some engine flush and an oil change before I'd go pulling the engine apart. Sometimes that can do wonders for a gunked up engine. Of course, it's your car, so you do what you feel is best. I'm basically a lazy bastard and want to do what's easiest.

Hi Guys,
First, let me say that I know nothing about the L cars- that being said, I had a similar problem on a car of mine, and it turned out to be low voltage output from the alternator. I was devilish to find it, but apparently, these computer cars are very voltage sensitive- a drop and they start to act up.
Don't know if this is at all helpful, but thought I'd mention it.

Hi NG51 and thanks for the suggestion. I checked the alternator early on and it read in the correct range. I would never have suspected it on my own but someone else mentioned the same thing.