What is the BTDC timing degree for a 4.0 LO enging?

[stonehands]

What is the BTDC timing degree for a 4.0 LO enging? Anybody know. Want to check timing but the book says it can not be adjusted and doesn't even give you what it should be. I have a miss and debugging step #4 is checking timing for me.

[Pete M]

The book is correct. Timing is not adjustable. It is set and adjusted wholly by the computer.

[stonehands]

So how do you know if is in time? Wouldn't you atleast be given the timing for idel at an rpm?

Could the miss be a bad injector or plug wire? What about a pluged cat? Ive posted before about this problem and the post got off subject and completly out of hand. I guess kids?

[Randy in Maine]

Try swapping around the plug wires or test the reistance of the ignition wires with your volt ohm meter.

 

A fuel pressure test may indicate a leaking injector, but it will not say much about a poor injector spray pattern.

[Geonovast]

I would do a whole tune up (plugs, cap, rotor, wires) before you worry about timing. The only way the timing could be off is if someone indexed the dist, the chain slipped, or something flat out broke. If the dist was indexed they would ALL miss if it even ran. If something broke you'd have a problem, with the 4.0 being an interference motor.

[Geonovast]

Hmm.. just checked the other thread and see you've done that. Replace the fuel filter, if that's getting clogged it could cause low pressure, or your pump is on the way out.

[HOrnbrod]

You can still check timing, but can not adjust it. There are timing marks on the harmonic nalancer and the timing chaing cover. They should line up with your timing light using #1 cylinder.

[FxRacing282]

should be >0 and <8 for an automatic. +2or so for a manual tranny. under acceleration you will see it advance a few degrees.

[stonehands]

Wow 2-8 BTDC. Mine is over 12.

Just found a leak in my cruze control vacuum line. Looked at this before and didn't find any thing. Now I know why. the leak was a brittle section in the line that goes down thru the headlight area. Vacuum test to come. Took her out after the leak fix. Actually found it when I moved it to the engine compartment from front bumber. kept hearing air flow in one side of ball. It has two sides one for heater controls and one for cruze. I digress. Stomped on gas at dead stop in rain. Launced me forward but didn't break loose. This thing gets good traction I guess. Power braked it and the tires lit up. Will be testing it out later for up hill power later. Will also check timing again after vacuum leak fix. Still a slight miss. Asuming it is injector issue now. Later on that. Thanks for the help so far guys making some progress here.

[stonehands]

Checked timing again around 19 deg BTDC. Any thoughts?

[Eagle]

2 to 8 degrees is too low.

 

The FSM and all the aftermarket manuals agree that timing is not adjustable on the 4.0L so they don't provide timing specs. But -- the 2.5L began life with adjustable timing and then it became non-adjustable when it went to the same Renix-based ignition as the 4.0L. The initial timing listed for the older 2.5L engines is 12 degrees.

 

Then we should keep in mind that the 4.0L is essentially the same engine as the older AMC 199/232/258 6-cylinder engines. For the 258, 1983 is the last year I have tune-up data for. The timing for that year is given in Haynes as 15 degrees -- and 19 degrees for the high-altitude models. Initial advance was less way back in the 60s, but I remember that I used to run my 199 c.i.d. I-6 at 12 degrees, which was more than called for but I ran high-test gas to eliminate the knock problem. I also got 28 MPG highway, which was the purpose of jacking the timing beyond factory spec.

 

With the Renix 4.0L, the initial advance can be pushed way up because the system has a knock sensor, and what that does is retard the spark when it sense detonation. In fact, the Renix system has a high-altitude CPS available, and that version of the CPS provides more initial advance than the "flatland" version. I've been running a high-altitude CPS in my '88 XJ for years. My house sits at elevation 475 feet above sea level, and I have no problems with detonation.

[FxRacing282]

2 to 8 degrees is too low.

 

The FSM and all the aftermarket manuals agree that timing is not adjustable on the 4.0L so they don't provide timing specs. But -- the 2.5L began life with adjustable timing and then it became non-adjustable when it went to the same Renix-based ignition as the 4.0L. The initial timing listed for the older 2.5L engines is 12 degrees.

 

Then we should keep in mind that the 4.0L is essentially the same engine as the older AMC 199/232/258 6-cylinder engines. For the 258, 1983 is the last year I have tune-up data for. The timing for that year is given in Haynes as 15 degrees -- and 19 degrees for the high-altitude models. Initial advance was less way back in the 60s, but I remember that I used to run my 199 c.i.d. I-6 at 12 degrees, which was more than called for but I ran high-test gas to eliminate the knock problem. I also got 28 MPG highway, which was the purpose of jacking the timing beyond factory spec.

 

With the Renix 4.0L, the initial advance can be pushed way up because the system has a knock sensor, and what that does is retard the spark when it sense detonation. In fact, the Renix system has a high-altitude CPS available, and that version of the CPS provides more initial advance than the "flatland" version. I've been running a high-altitude CPS in my '88 XJ for years. My house sits at elevation 475 feet above sea level, and I have no problems with detonation.

 

I'm impressed it is that high. but more timing means more power.. usually

[Eagle]

I'm impressed it is that high. but more timing means more power.. usually

Not really. All timing does is optimize the amount of power you can get from the gasoline you're burning.

 

The key is that you need the fuel-air mix to ignite at a time when the expanding gases resulting from combustion will be pushing down on the piston after it has passed top-dead-center, and not before. The combustion isn't an instantaneous event, and that's the problem. If the "event" starts too soon, the pressure starts to build before TDC and you lose power because some of the combustion is actually slowing the piston down as it rises toward TDC. In extreme cases, the expanding flame front slams into the piston destructively, with the flame front slamming down and meeting the piston as it's rising up (rather quickly) -- that's what we call "detonation," or "ping." Ping IS a destructive event, which is why we don't want to hear it -- ever.

 

On the other hand, if the combustion "event" starts too late, the piston will have gone beyond TDC and started down again, leaving a larger combustion chamber and resulting in lower compression. Some of the energy produced by the "event" will thus be lost -- or at least used less efficiently.

 

Now ... what most folks don't understand is that ALL gasoline contains the same amount of energy per unit of volume. Octane doesn't add any energy or power to the gasoline. All octane does is slow down the rate at which the fuel-air mix burns. What that accomplishes is that it slows or reduces that advancing flame front, allowing the timing to be advanced farther before TDC so that the event can begin earlier, with the goal being that when the piston reaches TDC the combustion event will be reaching maximum expansion, thus generating maximum power. Going back to my old 199 c.i.d. Rambler American, I don;t remember the "official" timing spec, but I think it was around 4 degrees. And it was for regular gasoline. By switching to high-test, I was able to take advantage of the delay in the flame front and dial in quite a bit more advance than what the actory called for.

 

The dealer who sold me that car was a family friend. My grandfather began buying cars from his father before WW2. I mentioned to him one day what I had done and what kind of gas mileage I was getting. He nearly had a heart attack. "Geez," he said, "don't let anybody hear you say that. My God, if anyone hears that ALL my customers will be coming in and asking why they don't get 28 miles per gallon."

 

So ... more timing might mean more power -- if the gasoline is properly matched to the timing spec. Too much timing advance with a low octane gasoline, though, will blow the tops out of the pistons in a VERY short time.

[FxRacing282]

right good point. but you can't argue that if you bump timing a few degrees and no pinging then you get more power. my truck spec is 8 degrees and i run 12. no pinging and awesome acceleration for 3.42 gears.

[mvusse]

Any vehicle with a knock sensor, which is the vast majority of vehicles with computer controlled timing and all newer vehicles, will retard timing if ping is detected. This includes our 4.0 engines.

 

In those engines, if you modify the distributor to advance timing, unless you run high octane fuel, the computer will just retard timing back to about where it was originally.

 

I tried it on my Suburban. Advanced 5 degrees from factory. Got a bit better better gas mileage on 93 octane, but same on 87, and not enough to offset the higher price per gallon. After 2 months it started to run like crap and missing. When I pulled the distributor cap, all of the contacts had burned away quite a bit from sparks when the coil ignited after the contacts in the cap were not closed.any more. New cap installed, timing back to factory specs and things are still good 3 years later.

[Eagle]

Any vehicle with a knock sensor, which is the vast majority of vehicles with computer controlled timing and all newer vehicles, will retard timing if ping is detected. This includes our 4.0 engines.

Correction - it includes some of our 4.0L engines.

 

The Renix system has a knock sensor. Chrysler deleted the knock sensor when they changed over to a Mopar injection/ignition system in model year 1991. No XJ or MJ 1991 or newer has a knock sensor.

[mvusse]

Thanks for the correction. Learned something new again.

[carnuck]

You can slightly modify timing by slotting the crank position sensor so it can move. The dist has a sensor that adjusts fuel spray timing so you lose power if it's messed with. Vacuum leaks cause missing too.