Lt4-396

96' lt4 has 41-43psi fuel pressure, has spark at plug and at the coil.
Drove for roughly 2 hours and at a red light car just shut off, got towed home and it started right up, shut it off 4 times and started perfect after those shutdowns.
Went to start it the next morning and it just cranks and sometimes stumbles a tiny bit at the end of it cranking.
Fuel pump is priming and has 43psi at the valve and has good pressure while cranking.
My scanner can't read the computer when it's cranking so I'm unsure if the opti is sending a signal to the pcm so tomorrow I'm going to get a noid light and see if the injectors are getting a signal from the pcm.

Has no codes or a check engine light.

I pulled a few wires off at the plugs and am getting a good spark, I also pulled the wire off the opti and am getting a good spark from the coil, I'm unsure if that rules out the ICM and opti?

I'm going to go threw the flow chart in the FSM this weekend but figured I would post here and if anyone has any input,what to check and look for it would be appreciated.

It sounds like it's ignition related since it does stumble for a half second especially since it does have fuel, I've read a bad MAF can cause a no start so I'm going to unplug it tomorrow and see if it starts.

Just very weird that it just died out of nowhere and wouldn't start then after the tow it started and ran fine and shut the car off a handful of times that day and it fired right up then the next day it was dead again, just cranking and cranking.

TIA

C409

..... Ohm check your fuel injectors ... cold and warm if you can get it to run long enough to heat up ... they should all be around 16 ohms ... if even only one is out of spec it can cause all to be/act faulty .....

Lt4-396

That's the thing it won't run at all now, I ohmed one of them I believe it was 14-16 ohm, I couldn't get the DVM to stay on a consistent number.
I will test all 8 of them tomorrow.
How far off can they be from 16? 16ohm +/- what?

scrappy76

You are saying it wont start now ? This would be a perfect time to spray some starting fluid in the throttle body. If it trys to start, I think you have a fuel issue. It wouldn't hurt to try.

haxxx

Testing ohm on injectors doesn't hurt but it is far from conclusive. If they are the original multec injectors more often than not they ohm perfect but have a bad spray pattern. I'm also in doubt that the injectors would cause it die after it's been running. You could have two completely non-flowing injectors and the engine would seem to be running just fine.

I've recently developed a problem on my LT4 where if it's been running for a while and I shut it off, wait 15 minutes and start it back up, it idles really low like it's going to stall for 10 seconds then it fixes itself. Only response I got was ECT going bad. Apparently the computer lives and dies by the coolant reading on this thing. Another possibility to consider.

aklim

IMO, it is only a good diagnostic IF the coil is shorted. If it tests good, at best, it gives you a meaningless result, at worst, sends you in the wrong direction. I kinda avoid it for that reason. If they are multec, I'd toss them as far as I can and shoot them. If not, might as well get them cleaned and tested or get a reman set for about $180 from FIC. The injectors get clogged or partially clogged and the ECM will adjust till it can adjust no more. I send mine off for cleaning and testing every 3 years and get some increase in flow per the before and after test indicating there is some blockage.

I'd hit the sensor with an infrared thermometer and see what the scanner says the ECM is seeing. The two should be relatively close. If not, that is something to replace.

Lt4-396

If the injectors are clogged why would it just shut and not start then an hour later run perfect and now not start.


It's not the ECT digital temp readings are where their supposed to be.
I will try the starting fluid but I'm 99% sure it's not a fuel problem.
The injectors are whatever came stock on a lt4, if I end up buying new ones I would go with the 30lb bosch injectors.

I don't know if what I'm hearing is a small backfire threw the intake or it just trying to run for a half second and running real rough for that millisecond.

Anyone else have a opinion or a suggestion?

Thank You,

Purple92

OK - Basic Auto Mechanics 101 - In order to run, a car needs three things:

1. Spark (timed more or less correctly)
2. Fuel
3. Compression

So - Hate to say it - but doing a compression test might be worthwhile - just to rule out crazy stuff like a broken camshaft or a broken timing chain.

If the compression is good (and there is a very high probability it will be) - it sounds like you have an intermittent problem with ether fuel or ignition.

I would definitely try checking to see if the PCM is commanding the injectors to fire by hooking up noid lights at the fuel injector connections. Shooting a little starting fluid into the air intake and trying to start it is not a bad idea either. It wouldn't be a crazy idea to pull one fuel rail and make sure the injectors are actually squirting fuel as the engine cranks. (And to make sure they aren't leaking badly and causing such a rich condition that the car can't light.) If you have fuel, and you see spark at one plug - you might just want to check for spark at a few different plugs - I've seen a post a long time ago where someone had a broken rotor, and was consistently getting a spark at the one cylinder he was checking when cranking.


GOOD LUCK !!!!

aklim

Grasping at straws here but it might be one of those cases where it isn't spraying the fuel properly and it works for some conditions but not for others? Thing is, they are tricky. It creeps up so gradually, it is hard to tell for sure till it is FUBAR. A perfect example is one of my injectors had a collapsed basket filter and the numbers were way off. About 30% or so. I couldn't tell a thing but the flow bench can. On my diesel car, I picked up 1.5 mpg by cleaning it at 100K. At 200K, I had to replace the nozzles because it wasn't spraying right and wasn't rebuildable. Either way, the car ran like it always did.

FAUEE

Get a new ICM. They get hot and then fail. With no ICM, you have no spark. When it cools down, you get spark again. I ended up redoing my whole ignition system because of a dead ICM (and a DOA replacement). It didn't hurt the car, but the car was down for a few months it didn't need to be because of it.

Lt4-396

I tried starting it at night as someone looked at the opti and wires and saw the opti to coil wire arcing.
Put a new one on and it started right up. Even though I was getting a good spark (what looked to be a good spark) at the plugs I'm assuming it either wasn't a good enough spark or wasn't consistently good.

I'm hoping the problem is fixed. The part that truly bothers me is this set of wires only has 25 miles on them, I'm using the old opti to coil wire for now until it gets a little warmer and I can custom make another one and Install it.

I drove it around for almost an hour with no problems and shut it off a handful of times with no problem as well.

I am also going to relocate the ICM and use a heatsink from heatsinkusa just don't know if I want it under the dash or under the hood somewhere.

Time will only tell...