2.3L ('83-'97) 1990 2.3 Odd Misfire Issues - New Engine

[Mr4btTahoe]

Hey alls,

So just put a fresh long block in my 1990. Everything is sealed up... fresh plugs.. coils and wires are only a year or so old and in good shape... same for the ICM. I replaced the CPS while the engine was out. Has a new o2 sensor... new intake temp sensor (looked nasty). Other than that, everything is the same.

It starts right up... runs glass smooth. Blip the throttle hard and it'll start misfiring... then it will smooth back out. Its much worse when cold. Once warm, it drives great... runs smooth... but if sitting at an idle and snap the throttle open hard.. it drops cylinders and smells pig rich... then it will clean up and run smooth again. When the engine is warm and the throttle is snapped hard... say 4k+rpm, it'll start running rough as it returns to idle... stumbling for a few seconds... then it cleans up and runs glass smooth again. Bring the idle up slow when its warm or during normal driving conditions, its fine as well.

When cold and it does this, it will stall at times... and the only way to get it to start back up is to hold the throttle wide open. Its flooding. Also when cold, it does it with barely any throttle at all until it warms up.

I'm assuming its a rich condition causing the misfire. I've checked all of the sensors... everything seems to respond when unplugged as it should. Unplug the CPS and it stalls instantly. Unplug each coil pack and it makes a noticeable difference but not the misfire that is happening. TPS causes an instant high idle... IAC will cause a stall when unplugged and warm (seems to be working fine... higher idle when cold... gradually drops as it warms up... etc.) No vacuum leaks that I can find... EGR seems to be fine... coolant temp sensor doesn't make much of a difference.

The truck has had a bad exhaust leak where the manifold meets the exhaust system. The bolts wont budge and the connection is loose. Could the new o2 sensor be responding faster than the old one... picking up the exhaust leak and causing a rich condition when the throttle is snapped?

I can't pull codes on this truck for some reason. It's always been that way though. Check engine light is dimly lit always... and if you jumper the diagnostic connector, it does nothing but cycle the fuel pump a few times. Its been that way for years though so it's not the cause of the issue.

Sorry for the long winded post but I'm really at a loss on whats causing this. Never had an issue like this one.

Engine was replaced due to massive oil leaks.... well sorta. Pulled the engine for a full re-seal and clutch job. Head ended up being cracked... couldn't find a good head and a fresh one was stupid expensive. The long block came with a 4 year warranty so figured it was the best option. This doesn't seem like a mechanical issue at all.

Appreciate any input.

 

[franklin2]

Unplug the O2 sensor and run it like that. See if it makes any difference in the problem. You might want to unplug the battery also to make sure the computer "forgets" anything it learned when it was not running right. Let it learn without the O2 sensor connected. If it runs better, I would suspect your exhaust leak is causing problems for the O2 sensor.

 

[Mr4btTahoe]

Unplugged the o2... same issue.

Unplugged the ECT on the bottom of the intake... same issue.

While its misfiring/stumbling... it smells really rich.

I unplugged the vac. line going to the pressure regulator... idle changes a bit but not much and the vacuum line is dry. Crank it over with the throttle wide open and it doesn't stumble/start.. etc.. so I doubt it's injector related.

Here is a video to show what it's doing. It gets better as it warms up. I'm stumped.

 

[scotts90ranger]

'90 is a break year, first half used a MAP sensor, second half used a MAF, which is yours? And as fun as it sounds it's also the same break year for the 3 and 4 wire oxygen sensors, and the 4 wire sensor will plug into the 3 wire connector so that makes it even more fun

If it runs better warmed up and smells rich before that that is pointing at really either the MAP or MAF sensor really, if MAP check the hose to it for any issues as that's important, if MAF clean it... When you say it runs better warmed up that says the oxygen sensor is working... Either that or your fuel injectors aren't working right or fuel pressure is too high or regulator isn't working...

 

[Mr4btTahoe]

MAF based yet nothing changed with the MAF during the motor swap. I can clean it though. I ran it earlier today with the MAF unplugged. No change in the misfire... very little change in how it was running.

4 wire o2 (as I bought the 3 wire based on rock auto and it was wrong..

If it were fuel pressure being too high, I'd assume it wouldn't idle well and run good at times. I can check fuel pressure though tomorrow. The idle does change a bit if I unhook the vacuum reference to the regulator.

Even with the o2 and ect sensor unplugged, in the short time it was running... it got better.

Once it's at operating temp... it takes an effort to get it to act up... will only do it if I free rev it passed 4k or so... then it stumbles and misses as it comes back down to an idle... and smoothes out within a second or so. Otherwise it runs good.

When cold on the other hand... touch the throttle and it starts missing... free rev it hard and it overfuels hard and will stall. Will only restart with holding the throttle wide open.

 

[scotts90ranger]

Unless the long term fuel trim values are wonky from the old engine (unlikely, I'm sure you had the battery out while swapping the engine...) there's something in open loop fuel control that isn't working right, the base tables are off, they're based on engine speed and MAF input assuming the fuel injectors are working right and the fuel pressure is in spec... unless you missed a ground or power wire going to the computer (one of the small wires going to the battery cables)

 

[Mr4btTahoe]

I'll check fuel pressure in the morning and go over the harness again. I'll clean the maf while I'm at it amd see what we've got.

Battery was unhooked for over a week waiting on the long block to show up.

So on cold start... the o2 isn't in play yet... its MAF only and rpm based?

How is ignition timing controlled? Is it all just off the CPS?

 

[franklin2]

On cold start it has charts inside the memory of the cpu that it uses to fuel the engine. Once it warms up it uses the O2 sensor to trim the fuel.

 

[Mr4btTahoe]

Update...

Tested fuel pressure.... 40psi koeo. 35-40psi while running... while running rough.. etc.

Tested the MAF... seems to be working as it should. Has good grounds... has good power signal... and signal output voltage is smooth and gradually increases as throttle increases... decreases smoothly as the throttle is closed.

Disconnected the battery and let it sit.

Same problem. Runs great when cold as long as you don't touch the throttle... touch the throttle and it goes rich and runs terrible. Once warm... runs great unless you rev it out hard... but when warm, it recovers much faster.

I put a timing light on it for giggles... timing jumps around a little when its running rough... but I think its due to it misfiring... not the cause.

I'm going to reinforce the main engine ground here in a bit... long shot but who knows.

I'm running out of ideas.

 

[Mr4btTahoe]

Update... reinforced the main ground.

No change. 0 ohms of resistance between the battery negative and anywhere on the engine itself now where before, it was ~3ohm or so.

Still no change. Went on and changed the break in oil as its had 40 or so miles on it and a few hours of run time. Oil was clean.

Grasping at straws here guys... no idea what it could be.

 

[RobbieD]

You've been pretty thorough, and I'd be tearing my hair out too, at this point.

Don't know if you've done this yet, but it wouldn't hurt to open up the PCM and look for evidence of leaking capacitors or signs of water damage. It's behind the passenger kick panel trim.

 

[Mr4btTahoe]

Truck ran great prior to having the engine out. It sat for over a week with the fuel system open. It's been suggested that I try swapping injectors as I could have one sticking. The injectors are original (although I did replace the orings on them prior to installing them... they looked a bit rough overall).

I may try and get a set of injectors for it to scratch that off the list.

I wouldn't think PCM as the truck has been sitting indoors the whole time.

 

[franklin2]

I put a timing light on it for giggles... timing jumps around a little when its running rough... but I think its due to it misfiring... not the cause.

I think this needs more investigation myself. I wonder what the computed timing test tells you?

 

[scotts90ranger]

The timing table is based on MAF reading and engine speed, except at idle there is some spark stabilization, if you look around there's a SPOUT connector with a little shorting bar, unplug it and the timing will stop moving around... If it is misfiring the load goes bonkers so the spark will move around, I'm not sure that is telling us much...

Might be a stupid question, but if it ran great before you put a new engine in, why'd you change engines?

Have you verified that all the plug wires are in the right spot? If you have 1 or 4 mixed with 2 or 3 on one coil that can do odd things... you can mix wires up front to back on the coils but not side to side...

 

[Mr4btTahoe]

The timing table is based on MAF reading and engine speed, except at idle there is some spark stabilization, if you look around there's a SPOUT connector with a little shorting bar, unplug it and the timing will stop moving around... If it is misfiring the load goes bonkers so the spark will move around, I'm not sure that is telling us much...

Might be a stupid question, but if it ran great before you put a new engine in, why'd you change engines?

Have you verified that all the plug wires are in the right spot? If you have 1 or 4 mixed with 2 or 3 on one coil that can do odd things... you can mix wires up front to back on the coils but not side to side...

Engine was swapped due to an oil leak... sounds odd... but it was leaking a quart every 50 miles or so. Pulled the engine to do a re-seal and clutch job. Oil leak was coming from the oil pan and the back corner of the head gasket. Pulled the head and it was cracked. Couldn't find a cylinder head... long block was the next best option as I couldn't find a good engine in my area. Long block has a warranty that pays labor so figured I'd bite the bullet. Everything else has already been gone through on the truck and its relatively rust free... couldn't get a good truck for $1800 in my area... so here we are.

I went through and pulled every connector off of every sensor and cleaned them. No change.

Cleaned the MAF. No change.

I ordered a set of injectors of Amazon... I'm going to swap them monday night. If it doesn't fix it, I'll send them back and put the originals back in it.

While it's running... I can disconnect each coil individually... it makes a running difference but it will still misfire the same no matter what coil is unhooked. So if it is spark related... it's gotta be the ICM as far as I can tell.... which wasn't having any problems prior to pulling the engine. I also put my inline spark tester on every plug... spark doesn't drop or change on any cylinder while the misfire is happening... which points to fuel or timing going haywire (which it doesn't appear do to).

My head hurts and I'm done for the night. Any more thoughts?