Ok, after countless hours in the garage trying to brainstorm and come up with the possible reasons my car is not starting, I have run out of ideas. I put in a rebuilt LS1 last summer and never got it to start up. Here is the problem:

When cranking, the engine will crank without firing for the first couple cranks. Then, it will start to fire in each cylinder, one every couple strokes. Once I have been cranking it for a little while, there is firing in all of the cylinders, but its very rough and it wants to die out. The longest I can get it to stay running is only about 2-3 seconds, then it will cut out. It will also not start at all when I just touch the gas, as if more air is making it cut out completely. Right when I start it, it will fire a cylinder and a few others with no problem, after that it will just die out slowly.

I have a custom harness and re-flashed computer from Speartech, but before I got the harness, I had the stock harness and un-flashed computer and it was doing the same thing. I know my harness and computer are good. I checked compression on all cylinders and it all checked out ok. I checked my grounds, all the sensors are plugged in, I checked plugs and plug wires, all looked good. When I checked the plugs, they were somewhat dark and a little wet. I am sure all my wiring is good, cause I get spark on all cylinders, and all my injectors pulse. Each cylinder does fire and gets good spark. I also checked to see if it was reading any codes, and no problem codes came up.

I am out of ideas and am getting depressed because so much time has gone in, and its still not working.

Please, if anyone can help pinpoint this problem it would make my year. this has been the one thing that has kept this car from being on the road. Please, let me know what your thoughts are and if you have any suggestions.

Thanks,

John

 

whats it in? fbody? it sounds like a vats problem or a bad crank sensor, check the fuel pressure.

 

Its in a 73' datsun 240z. I had vats disabled when the computer was re-flashed, I tried unplugging the crank sensor and starting it to see if it made any difference, but it didn't want to fire at all when I did that. Could the crank sensor be bad but still work and mess up the firing? As far as the fuel pressure, I have it exactly at 57 lbs, right before the fuel rail.

 

Incorrect pushrod length can cause it not to fire too....

 

is there an easy way to determine if my pushrod lengths are ok without disassembling the whole engine? I don't think its that because it fires, it just slowly dies out after a couple seconds, and won't rev up at all.

 

Yes, remove the pushrods and use a pushrod length checker ($20) to make sure you have the right length. To remove the pushrods, take off the coil packs, valve covers and remove the rockers. To check for the correct length, here are good links:

https://ls1tech.com/forums/generation-iii-internal-engine/748577-can-someone-help-me-pushrod-length.html
https://ls1tech.com/forums/generation-iii-internal-engine/749143-how-do-you-check-preload.html

 

If the pushrod length was off, wouldn't that make the compression off? I checked the compression and it was good.

 

Well, it could be 150-180 with incorrect pushrods and 210-220 with the right ones.

The only reason I suggested this is I had this problem in 05' with a new motor. Something for you to check and eliminate as a possibility.

Give us more info on the rebuild.

 

not always, there was another thread like this few months ago and it was push rod lengths, try doin a search for crank but no start.

 

thanks guys, this adds another thing to my list to check.

As far as info on the rebuild, the builders didn't build it to stock specs, and I don't know how reliable their work is, cause I haven't talked with anyone else that has used them, but I have heard some things that would make me not buy from them again.

I just got back from getting the alternator tested and it checked out ok. Any other suggestions other than the pushrod lengths? I will check those out once I pick up a length checker. Anything else it could be if the pushrods measure out ok?

 

Thought I'd note what was used in the rebuild. The engine is using new competition lifters, and chrome moly pushrods. Not exactly sure on the measurements. The builders were a bit vague on the details, so I can't really through out any numbers. Any other ideas other the the pushrod lengths?

 

Well, the preload could be off due to what lifters they used. Too much preload and an engine won't fire; that was my case.

 

alright, so I was just in the garage for a while playing around with the sensors and vacuum lines. I disconnected the vacuum line just behind the throttle body and blocked it off. Then cranked the engine again and it started right up and ran smooth for a solid 2-3 seconds with proper firing then it cut out. I then connected the vacuum hose back up and started it again and fired, but had a very loppy idle for about 5 seconds. I am wondering if it is an air problem or something having to do with vacuum possibly.
Any thoughts?


Viper, do you mean it won't fire at all? or it just will fire, but not want to run? The engine is firing, but it just won't stay running for more than a couple seconds.

 

Yeah, won't fire at all. Since your is, I'm starting to rule the pushrods out. I'm hoping you get more people to chime in as I'm out of ideas my friend.

Here's a good link with some ideas to try:

https://ls1tech.com/forums/showthrea...es+runs+engine

IAT and making sure the VATS is truly disabled is what I'm gathering from the thread.

Here's another that deals with the alternator; seeing as how your a conversion it might be worth looking at:

https://ls1tech.com/forums/showthrea...es+runs+engine

Yet another with VATS:

https://ls1tech.com/forums/showthrea...ns+engine+dies

Of course having tuning software helps with some logs. What are your IAC's at? That will help us determine if it's an AIR problem. Granted, you'll only get 3-5 seconds worth but still...

How old is the gas in the tank? It could be bad if it's been over 6 months.

John

 

Viper, thanks for the links, I read through them and the one I am looking into is the third one listed. It sounds just like my problem, sent the guy a pm.

As far as the vats, I sent my computer into Speartech to have them flash it for me, so I am almost positive they disabled vats when they were programming it.

The IAT is plugged in, but I was thinking even without it plugged in the engine should still not cut out if it was starting up?

I took in my alternator yesterday to have it tested, and it was good, I also tried starting it without it plugged in and it fired the same way.

The gas in the tank should be ok, I have put it in this summer and it was somewhat fresh then.

The thing that was on the back of my mind also was the IAC. I took it off yesterday and played with it, when its pushed in and you plug it back into the harness and power it up it moves back out, but I was wondering could the IAC still work, but be faulty? As in, would it not regulate the air correctly but still move around? cause it seems like it is cutting off air into the manifold, but the IAC is the only thing that would do something like that right? Then again, even if the IAC was the problem, when I would open the TB just a tad, it shouldn't cut out completely, should it?

 

" The gas in the tank should be ok, I have put it in this summer and it was somewhat fresh then."

It's pretty old by now, like a good 6 months no?

"Then again, even if the IAC was the problem, when I would open the TB just a tad, it shouldn't cut out completely, should it?"

I agree but worth replacing for as cheap as it is.

 

Yea, those are two things I haven't done yet, and since I am out of things to try, I will give anything a shot now. I'll flush out the old gas and put new stuff in, wouldn't hurt, and gas is still a cheap fix.
As far as the IAC, its worth trying to replace cause if it doesn't fix the problem I'll just have an extra one for future use I guess.
Viper, would you happen to have any spare IAC's laying around? If not, cheapest place to buy em'... gmpartsdirect.com?

alright, so I was brainstorming a little and this is what I was thinking...
If I do have all my vacuum lines and air hook up right, it should start, even if the air sensors weren't all connected right? I know it would run horrible, but it probably wouldn't cut out every time... Because its cutting out every time, the same way (just dies out), wouldn't it make sense that its a fuel issue then? But if I am getting all the cylinders to fire, all my injectors are working, and I'm getting good spark. My fuel pressure stays up when it fires and runs for those couple seconds, so fuel pressure is adequate.
I think I am just going in circles, brainstorming and coming back to the same issues...

Is there anything that would either seem so simple, or so out of the ordinary that I could be over looking? What about vacuum lines? Would having any of the vacuum lines not connected or blocked off keep the engine from running? When I played with the vacuum line just behind the TB and blocked it off (the one that runs to the valve cover) the engine fired up and ran the smoothest I have heard it run for about 2-3 seconds, then cut out.

Any thoughts guys?

 

Should we start a little competition? $40 paypal'd to whoever can figure this one out? eh?

 

go back to the basics ....... a motor needs compression, fire and fuel to start , run a compression check in each cylinder... If you have all three, id say that is is proly your crank sensor

 

I thought a crank sensor wouldn't allow it to fire. And if it did, only maybe one cylinder every several cranks. I did a compression test and it checked out ok. I know I have fuel, and spark cause I can get it to run for about 2-3 seconds, but it cuts out after that. I think I will try disconnecting the MAF and starting it, I have heard of peoples engines not running because the MAF was faulty and it was plugged in... not plugged in it ran, but ran poorly.

Could it be possible something is arching to something grounded somewhere? Would it even allow it to start up? if arching is possible, what is a good way to test if my voltage to ECU and around the engine is good? where would the best location be to test it at? wiring to the ECU?