Akward start-up issue (searched!)

ASSISTANCE Stock Engine-Bay Components
1

Ive been helping a friend recently after swapping in a new LS into his car. Its been having a lot of trouble starting up, but if you use something (carb cleaner, etc) to coax it, it runs fine. Ive run through all of the basic testing that you can really do (as far as i know) and im really stumped. heres a breif run-down of whats been done since i started the job.

#1 - check 12v system, everything is working as it should, starter cranks, battery is strong, etc.

#2 - wires looked a bit sad, pulled cap off distributor and found the rotor was VERY badly worn, replaced plugs, wires, cap, rotor, and fuel filter.

#3 - with new tune up installed vehicle still cranks and cranks until given a little extra love to kick over, but once it does, it idles perfect, revs perfect, etc.

perhaps im just overlooking something simple here, but i have air, i have my fuel, i have my spark, its just the initial kick over of the engine that doesnt seem to want to take. ive searched for two days to try and find anything relating to this, maybe i missed it if its here, but i sure did try.

Thanks for your help in advance.

EDIT: just to let it be known also, vehicle has a new fuel pump installed, and the MFR has been swapped with a known working unit. (from my vehicle which runs fine). Thanks again.

2

You’re getting fuel and spark, but is it quality fuel and spark? That’s what I’d be asking myself. What was the history of the engine? Was it pulled from a running car? Something that was just sitting? Thinking about those things could help narrow down where to look for your problem.

Have you checked fuel pressure? What about injector spray pattern? How’s the compression?

3

Colin, i apologize, i was a bit hasty to post. As for the engine history, pulled from a running 91 LS (ran less than a month ago, the swap took a bit longer than previously thought, so it has sat for a month) Compression is strong across all 4 cylinders, 220k miles on the engine. As for fuel and spark quality; Spark is very strong across all 4 cylinders since the tune up parts were installed, so im leaning towards fuel. we have good fuel pressure according to the helms, but i havent checked injector spray.

Its just baffling that it ran, nothing has been changed, including the same ECU, harness, etc. all sensors are installed and hooked up appropriately, and we have no CEL or code issues. Hopefully this info will be of any service to advice on where to turn next.

4

Yeah, your concerns seem to be in alignment with the situation. Definitely doesn’t seem like a wiring error or really anything you guys did wrong. My guess is that it’s something completely coincidental. I can’t tell you how many times I’m working on the car, run into a problem, then end up wasting a TON of time assuming that something I’m working on has caused the problem. Only to find out that the problem was completely unrelated or at least not caused by anything I did - it was just time for that part to fail.

5

One concern i seem to have run across from numerous threads i have found while searching, pointed towards the ignitor, i think it seems like a long shot considering it runs once you give it a boost with the carb clean.

With everything said, I think I might just break out the multimeter and run some tests on things like the ignitor, or coil, just on the off chance that maybe its a bit weaker than i think it could be? Also, as far as i know, backpressure shouldnt really be a factor in initial startup? Right, or am i mistaken on that?

6

My experience has been that taking readings on those distributor parts isn’t very helpful. If they’re wildy out of spec, then yeah, it’s obvious it’s bad. But I’ve had them be bad and still be questionable regarding whether they were in or out of spec - same with parts that are known good.

You mentioned you tried your MFR on his car… why not swap your distributor over? Assuming they’re both the same OBD.

7

hmmm. strange. how does it run once it’s going? maybe start with the basics. check timing…

8

Colin: sadly i cant swap distributors with him, so that is a bit of a problem to test those issues. I understand what you mean about the ‘worn’ ‘not worn’ parts issue, its just a thought since i have the ability and time, you know?

welfare: once she fires over the car purrs like a kitten, absolutely no skips, no misfires, nothing. idles right around 800rpm, revs up to the limiter like its brand new. timing belt was recently done and is still in time, havent jumped any teeth or anything.

Anyone else have any insight? Im stumped!

9

masshole,

I haven’t experienced that before, but I could see if the fuel wasn’t atomizing well, it causing issues starting like that (which Colin already hinted at about with the injector spray pattern). I would make sure that the little filter screens in the injectors are all clean. I’ve actually never seen more than a few specs of dirt in them, but maybe a spider crawled into the fuel rail while you were doing the swap. You never know. A good scrubbing may do them some good. You can also check the resistance of the injectors themselves, but since the car runs fine afterwards, they will probably check out fine.

If you think it might be the distributor which, I’m with you, doesn’t seem likely, you could always buy a new one from an autoparts store with a good return policy and then see if it fixes things. I’ve found that a lot of those parts ‘just don’t fit right’ :wink: Sure it may be a little morally questionable, but I buy plenty of brake cleaner from those guys to make up for it…

10

I may be able to get you a free distributor if you think you may need one. I sent you an email already regarding this, but I figured I would post here just incase you do not check it all that often.

Miles