Poor running/misfire

[SpeedAddict62]

First off, I'm new to DSMs. I acquired my car recently, its a 1990 GSX and its completely stock.

My problem started when I was driving on the freeway and I noticed an intermittent stutter. The car stopped accelerating. By the time I got off the freeway, there was a noticeable loss of power, and the engine would barely idle. I haven't driven the car since, but it will start and idle, it just runs poorly. Spark is weak, especially on the #3 cylinder, which seems not to be firing. The idle is lumpy and unstable, it will get up to normal (700-1000 depending on temp) and then sometimes drop to 500 or so. The spark plugs are new. I have searched, and from what I have read it could be a weak coil or the CAS. Am I right? I plan to test the resistance of the coil today, once I get my hands on a multimeter. Any suggestions? What should be my next step? I hope I gave enough information here, but any help would be greatly appreciated, I want to get back on the road.

 

[Whammydill]

Make sure your plugs are good, sounds like they are. Make sure the gap is correct. You should be using NGK BPR6ES I think for stock. .028 gap.

Check/replace the plug wires, make sure they are seated properly at both ends.

get/install a mechanical boost gauge.

Do a boost leak test.

pull your ecu out and check it for corosion, the capacitors in our cars are notorious for leakage.

You can also try manually adjusting the idle speed and see if it helps, might not though if there are other things messing it up.

if you don't know already, go here for information on how to do most of this http://www.vfaq.com/FAQlocator-index.html Also check the links that come up on that page, good info there too

Check tech articles for the other things.

 

[SpeedAddict62]

I pulled the coil yesterday and tested it with a multimeter, and all the readings were in spec (.8-1 ohm primary, 12.3k ohms secondary). I tested the spark plug wires just for fun and I was surprised when they only read 1k ohms. I will replace the wires (I have no idea how old they are) and I'm not sure what I will do about the coil. Today I will pull the ECU and take a look, and then I will build a boost leak tester. The spark plugs are NGK BPR6ES at .028 gap, just like you said.

 

[SpeedAddict62]

An update:

1. This is what I get for not checking the simple things first. I didn't pull the plugs until today, because they only have 400 miles on them. Bad idea: half the electrode has melted off the #1 spark plug. #2 looks like it was beginning to melt, #3 and #4 look fine.

2. Apparently the last owner thought it would be a good idea to prevent the capacitors from leaking, so the middle of my ECM is completely covered in some kind of silicone sealant.

3. Today, I accidently dropped the coil pack, breaking one of the posts.

So now I need a new coil pack, spark plug wires, and spark plugs, and I have no idea what would cause #1 and #2 cylinders to get hot enough to melt the electrodes. Apparently the ECM still works. I'm getting my injectors cleaned at the moment, just to be safe. I do have an APC boost gauge now (I haven't heard anything good about them, but hey, it was a present. I can always upgrade later.) but it isn't hooked up yet. I've thought of trying some colder plugs and checking my fuel pressure to make sure everything is up to spec and not going lean. Any other ideas/advice?

 

[Whammydill]

Well, at least you caught things before they got too bad. As far as I know, the nickle alloy in the plugs melts at about 2300 degrees Farenheit, +/- 100 degrees. Thats pretty hot. Sounds like a lean condition but as to why its only the 1 and 2 cylinder (so far) I'm not sure. perhaps the sparkplugs in those cylinders were improperly seated.

The page below has some general info on sparkplugs with some pictures of plugs that have various things wrong. Compare yours and it may give you an idea of where to go next.

http://www.autohausaz.com/html/spark-plugs_spark-plug-wires.html

As far as the ECM goes. When mine went bad, it still "worked" that is the car would start and drive, but the ECM would often go off on its own and give real bad idle surge, would often stall when I press the clutch pedal in, (thought it was the Idle speed motor but it hasn't happened since i replaced the ECM,) I couldn't get the engine past 5k rpms (on a good day,) horrible misfires and stuttering. When I examined the ECM, it was evident there was some corrosion to the board from leaked capacitors, but it didn't look all that bad. But apparently it was all the difference. From what I can tell Steve(a wiseman) is the man to talk to about ECM's maybe he can tell you if having silicon on your board is a no-no and causing your problems.

 

[SpeedAddict62]

First off, thanks for your help Whammy, its greatly appreciated.

I installed a new coil pack today, along with a range colder spark plug (BPR7ES gapped at .028). I started the car up, and the result was the same as before. The idle surges anywhere from almost stall to 1500 rpm, missing the whole way through. My problem does sound very much like when your ECM crapped out, although I can get it to rev up to redline. A thought: Could (or is it likely) that the ECM could cause #1 and #2 to go lean?

 

[Whammydill]

quite possible. Quick check on that would be to find someone with a car with a compatible ECM and buddy up to them real nice so they let you borrow their ECM for a test. I think the ECM you need is number MD145903.

You might wanna check the fuel filter too. A dirty one can starve your engine im sure.

 

[SpeedAddict62]

Ok, now for an update:

1. I removed the ECM, removed all the RTV and had a friend replace the capacitors (just in case) for all of $3. There was no corrosion that I could see on the board, and when I put it back in the car it changed nothing.

2. I just installed a new fuel filter with a fuel pressure gauge. I also installed an Autometer boost and a/f gauge recently. The car does run a bit smoother now with the new fuel filter. I am running about 38 psi fuel pressure, 10 in Hg and the a/f gauge indicates a constant lean condition at idle. I hooked up the boost and a/f as per the vfaqs and searching around here. Assuming I hooked up the gauges correctly, I find both vacuum and a/f really odd, because I should be more like 19 in Hg and the a/f gauge shouldn't show a constant value in closed loop operation.

 

[tylermartindc]

just a thought, i know it may sound dumb but double check that ## o2 sensor is plugged in, or check the condition its in aswell. i was helping a friend out a few days ago and we forgot to plug it back in. this caused a constant lean condition showing on his a/f gauge.
just a noob tryin to help out. my .02cents

 

[Whammydill]

$3 worth of capacitors will save you hundreds later if you hadn't im sure.

And thats an excellent recommendation on the o2 sensor, I always forget stupid stuff and it costs me hours and sometimes $ afterward.

as far as the boost gauge goes, with my lack of mods, my boost gauge shows 20in/hg at idle, 24in/hg on decelleration, and around 12psi max boost. Your only showing 10in/hg at idle? Make sure your hose for the gauge don't have any kinks in it. And if at all possible, if its the nylon stuff, replace it with vac tubing or something more kink-resistant.

If your vac reading is true, then that will be a good a place as any to start, i'll see if I can find the vacuum translation page and post it back, at any rate it will all probably lead back to the almighty Boost leak test.

 

[SpeedAddict62]

First off, thanks for all your help, guys.

I borrowed a compression tester today and gave it a shot. Here are the results:

#1: 140 psi
#2: 140 psi
#3: 0 psi
#4: 160 psi

See the problem here? So I'm taking the car in to a guy I know, and I'll let him handle the repair, because the damage is beyond my mechanical skill to repair. It sounds to me like a valve but I guess there is no way to tell until he cracks open the head. This would also explain why #3 wasn't firing. Still not sure why #1 and #2 went so lean, but I guess this engine has more than one problem. I will post more when I have some answers.

 

[SpeedAddict62]

An update:

I was going to have someone pull the head for me...but I did it myself. There is a nice chunk missing from one of my exhaust valves, and I have no idea where it went. I'll take the head in to a machine shop, and get it rebuilt. I'll be replacing the valves/springs (not sure what brand) and looking at buying some comp cams (I can get them really cheap). I'm really not sure what combo to go with on cams; I really don't care about idle, I want some more power up top, but I don't want to lose too much down low, plus I'm still running a stock 14b (I do see a Evo 3 16g in my future though...). I'll also be installing a 3" exhaust and upgrading the fuel system while everything is apart.

I suspect the old fuel filter contributed to going lean, the car actually ran better with the new filter, even with a hole in the valve. I still want to upgrade to walbro 255 and -6AN lines.

Thanks to all who posted and helped me out.

 

[SpeedAddict62]

So I am bringing this thread back, here is my question: What is the likely cause of the spark plug #1 and #2 melting, and the #3 cylinder exhaust breaking like it did? I know this is an indication of going lean (and too much heat), but could that really be caused by something as simple as a clogged fuel filter? The fuel pressure is where it should be, around 38 psi or so (that was taken after I replaced the fuel filter). I just don't want anything to happen to that new head...

Here are a few pictures, enjoy. I still don't know where that valve went. And the other picture shows my reason for buying an EVO III manifold.

 

[BISHILVR]

It must be a 90 gsx thing because mine just did the exact same thing with the exact same symptoms. Finally did a compression check and zero on number two! The other plugs were white, it would misfire on loads only and if you would back out it would clear up, then the other night it just stayed a constant misfire. This car has'nt ran very good for at least two years though, so it might have been in the process for a long time.