Slight stutter misfire issue; need professional input

[3aG1e_7a1oN]

Stock 1990 Eclipse GST with 197xxx and still shows a lot of life. I've searched hours and hours on the forum and have spent hours and dollars on this issue with my newest DSM.

Now as a side note i bought this car from a crack head mechaninc in Topeka who put a 420a fuel pump in it. I drove it home probably about 200 miles with that pump and had no problems. The car ran like a beast when I got it home. I loved it.

Then one day me and my dad changed the alternator belt and after we changed that belt the car started slowly losing power. A day later I couldn't make it up the slightest hill and I couldn't go over 40 going straight. And ever since the car has been sitting as I replace and replace and drop money on an issue that is seemingly impossible.

I don't think it's anything drastic such as bad seals or cracked block. At idle the car will sit and be missing but if you crank it up to around 2500 it'll catch and rev up even higher to around like 4500 then drop back down to idle. But the whole time after 2500 while its revving and the misfire is not even there. Misfire at only low RPMS.
Here is what I have done:

• NGK Plugs - of course gapped correctly
• NGK Wires
• Replaced Coil Pack
• Replaced Alternator
• Replaced ECU
• Replaced fuel injector lower seals
• Tested CAS - Tested checked out ok
• Tested Injectors - I listened to each one with a pair of those doctor things checked out ok
• Boost leak test - found some small leaks but it checked out okay

At this point it runs real strong, but just real strong with a slight miss/studder.
I Just really need some feedback i don't know why my baby is doing this I just want to get this issue solved so I can start building.

And another side note:
This DSM was sitting for years at one point before i bought it and there are signs that rodents used to live in the engine bay. It was obvious that there were rodents however i have not found any chewed wires at all. There may be one wire somewhere that a stupid rat bit and is causing my whole damn problem.


Any input will help.

 

[And_44]

Have you checked vacuum hoses? Cheap to replace. Have you tested the ignition module as well? Fuel injector relay thingy on firewall. Verified that your getting enough spark to plugs? I ran into an issue when my alternator wasn't putting out enough power to fire my plugs fully and was having a misfire studder problem. Replaced alternator and fixed it. I know you already did but alternator from the common parts stores are know for not being of the best quality.

 

[dsmloverguy90]

see what your engine is putting out for vacuum and a compression test. i would start there being as how you get rid of the miss after 2500.

 

[3aG1e_7a1oN]

Have you checked vacuum hoses? Cheap to replace. Have you tested the ignition module as well? Fuel injector relay thingy on firewall. Verified that your getting enough spark to plugs? I ran into an issue when my alternator wasn't putting out enough power to fire my plugs fully and was having a misfire studder problem. Replaced alternator and fixed it. I know you already did but alternator from the common parts stores are know for not being of the best quality.

Thanks for the quick reply. Yeah I had your thread about your alternator bookmarked for awhile, but I replaced almost everything that was mentioned in that thread . I will check the things that you listed and get back with my results. Glad to see you still come around here to help .

 

[3aG1e_7a1oN]

Have you checked vacuum hoses? Cheap to replace. Have you tested the ignition module as well? Fuel injector relay thingy on firewall. Verified that your getting enough spark to plugs? I ran into an issue when my alternator wasn't putting out enough power to fire my plugs fully and was having a misfire studder problem. Replaced alternator and fixed it. I know you already did but alternator from the common parts stores are know for not being of the best quality.

Update: Finally got some nice weather in Omaha. Took until the end of May for the snow to melt and have 50+ degree weather. So I checked the vac hoses, replaced a couple that were needed. Fuel injector resistor pack is fine, I am getting enough spark to the plugs. It helped slightly.

Then I unplugged my MAF. The car started running like a champ. Better than I have ever seen it run before! Boostin me into almost another dimension. The car runs like crap when first started off, throws a CEL, but drives great besides a shaky idle.

Can anybody explain why unplugging the MAF would make the car run better, but slightly miss when plugged in? Again the car is all stock so it's just the standard K&N aircan with no mods.

 

[PEGE]

Update: Finally got some nice weather in Omaha. Took until the end of May for the snow to melt and have 50+ degree weather. So I checked the vac hoses, replaced a couple that were needed. Fuel injector resistor pack is fine, I am getting enough spark to the plugs. It helped slightly.

Then I unplugged my MAF. The car started running like a champ. Better than I have ever seen it run before! Boostin me into almost another dimension. The car runs like crap when first started off, throws a CEL, but drives great besides a shaky idle.

Can anybody explain why unplugging the MAF would make the car run better, but slightly miss when plugged in? Again the car is all stock so it's just the standard K&N aircan with no mods.

Perhaps trying an other MAF to see if this resolves the issue.
Borrow from a buddy if you can instead of bying to find out there was no isue with yours.

Maybe previous owner hacked the MAF wrong?

 

[Colt4G63 also]

Sounds like you need a new MAF. The old one was bad, but the ECU didn't know it was bad, so it continued to trust it's signal. WIth the MAF unplugged, the ECU knows it's unplugged, and runs in a limp mode. Don't drive around like that.

 

[3aG1e_7a1oN]

Okay. Does it have to be a 90 MAF or can it be a 90-94?

 

[3aG1e_7a1oN]

I have replaced the MAF with a known working one and still have the same problem. With the MAF unplugged it runs like an economy car but doesn't push any boost. With the MAF plugged in, it stutters/shakes in gear but not in idle. What is wrong with this POS?