Slow start

I did another compression test since I've had the xj for a while and the numbers are better. Avg 130 from around 125 I think. It's taking 4-5 cranks to finally fire. I have 42psi when cranking and 32 at idle. Replaced the regulator with no change. The only odd thing is that I poured a bit of seafoam into each cylinder when putting plugs back in from compression test and it fired on the first revolution.
Any ideas?

Ideas? What is the question, if any? Your 4 to 5 engine cranks to fire up seems good to me. A splash of SeaFoam into each cylinder would; a) increase compression a tiny bit, b) introduce a higher volatility, say similar to introducing 'ether', or any other 'cold-start' recipe, (which is never used in a diesel engine as such could crack the heads, etc., due to their tighter, and higher compression builds).

My question is why are your numbers better? Why was there a better change.., due to what, and when? Just curious...

I want immediate start. I pulled my Harley out after 5 months of hibernation and it started on first crank. Would love the same result on an engine being run almost daily. I'll be doing the injector swap soon and maybe that will help.

The numbers came up for a couple reasons. Seafoam and a couple oil changes. I fracked a bunch of carbon and I know now the po lied about maintenance so in the time I've had it, I've been able to bring some life back.

Define 4-5 cranks. Is that 4-5 times you turn the key to crank for a few seconds and then repeat?

Many people think that what they hear when the starter is engaged and it makes a rrrr.....rrrrr.....rrrrr....rrrrr. sound, each rrrrr is a complete engine rotation. What you are hearing is sequential cylinders going through a compression stroke. The computer needs to see the crank rotate to the point where the camshaft sensor and the crankshaft sensor can tell it where TDC is, before it starts the spark. The Renix book says it needs at least three cylinders of cranking to get the sync from the crankshaft position sensor, and then it uses the camshaft sensor as well to tell what stroke the engine is on.

I say all of this to let you know, the vehicle will rarely take over one rotation to start to fire up. It may sound like more, but it isn't.

It's probably only a 3 second wait but it's certainly spinning the cam a bunch of times because I'm hearing the pistons slap. It's always a wait of about the same time. I've never had an engine take this long that didn't have a problem. I would suspect it has to be a fuel delivery issue since adding seafoam to the cylinders made it fire up immediately. Would be a really strange coincidence that after almost a year, the only time the CPS was aligned to find TDC immediately was just after a compression test when I added some seafoam.

Thanks 0ld_man for the clarification, and for being there for us all.

Me, I always do my eye-ball compression check with a screw-in dial-compression gauge after the forth second on my wrist watch, or longer sometimes--all depends.

Three seconds to start engine seems ok to me, and that is about what it takes for my RENIX. Seems a bit slow compared to some other engine/systems I've had since 1964, but I figure it is ok, since it is the nature of these beasts. In 1972, my VW Van, (running on C3H8 fuel), started in two seconds.

When new it only took one, or two string pull starts on my 49cc two-stroke to fire up in order to motivate my mountain bike. Harley it ain't, but certainly a thrill, (laughing out loud).

EASY. Buy a russ pottenger stroker, and an odyssey battery. Starts on the second crank...

130~PSI compression is low... you mention piston slap... your sig says its a 91 4.0.... what do you expect from a 25 year old engine, that wasnt cutting edge WHEN IT WAS NEW!?

dan1977p said:

I want immediate start.

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Heres a question, does it seems to be cranking as fast as a normal XJ?

When starters take a sh!t, sometimes the start to crank slow and get weak first, giving you a longer harder start. The more amps the starter pulls, the less there are to go around (and do things like jump the gap of 6 spark plugs)

Anyway, listen to the next couple 4.0s you hear start up in a parking lot, or listen to a buddies, or just do a starter amp draw test.

Just my .02¢

I did a starter test at O'Reillys and it was fine and my optima red top six months old. I haven't refreshed all my grounds or cleaned my sensors/injectors. The thing that gets me is that fuel pressure is good cranking but low running. Replaced regulator to fix that. Even before that, a few cc's of seafoam in each cylinder fired it right up. I've been trying to catch some XJ's on the street but everyone seems to be in a hurry. I'll get a video of it today and post for others to hear to see if I'm just chasing a red herring.

Or if the batttery voltage drops under 10 V while cranking, find the problem.

Renix jeeps, 87-90 are well know for 3-7 second start times. The ECU does not fire the injectors until it gets all the data it wants, and that is a slow process. So no fuel is the design in the first seconds. Not sure if 91 was any different yet. 96 was much better.

If the 91 fuel rail/pump/regulator is like Renix then " I have 42psi when cranking and 32 at idle" sounds OK, but I think in 91 they went to 49 PSI (high output engine year change) and a regulator in the fuel tank? Is your's a renix in a 91 or something?

Ecomike said:

I think in 91 they went to 49 PSI (high output engine year change) and a regulator in the fuel tank? Is your's a renix in a 91 or something?

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...Nope, H.O.'s use a fuel rail mounted fuel regulator... ~98's+ started the fuel tank located regulators... Differant than the renix rails....

my renix XJ had a couple of periods where it would long crank before starting. lately i have been fighting high idle, but i had other symptoms along the way,one being getting hot and not restarting..i think that problem was my ignition module under the coil being on it's way out. common symptom and common problem with that part. it now starts in less than 1/4 of the time it used to. very near instant as soon as you turn the key

The starter test off the truck won't tell you what the starter does under load....only that it starts up and spins.