Why's it take so long to start a 4.0?

[Pioneer89XJ]

Yes the renix ignition/fuel injection sys on pre 91s requires a slightly longer crank time to fire up. Usually a count of from 2 to 5. Apparantly this is programed into the ecu for reasons not always clear to us. However a side benefit - more oil is pumped during cranking before the start up. Greg

True. My 89 took much more time than my 98 does.

 

[TheBlueSub]

i kno this is an old post but my 1990 takes a good while to start even when i turn the key to acc and let the pump prime...my '85 2.8 took a .5 of a second to start


does anyone else know any other solutions??

 

[90 Laredo]

I asked my local dealer and they said that some of the 4.0L had a delay built into the computer. This is done to let the oil pump to pump a bit to help eliminate dry start and prolong engine life.

 

[langer1]

I asked my local dealer and they said that some of the 4.0L had a delay built into the computer. This is done to let the oil pump to pump a bit to help eliminate dry start and prolong engine life.

Your Dealer was blowing smoke up your tail pipe. :laugh3:

 

[langer1]

Yes the renix ignition/fuel injection sys on pre 91s requires a slightly longer crank time to fire up. Usually a count of from 2 to 5. Apparantly this is programed into the ecu for reasons not always clear to us. However a side benefit - more oil is pumped during cranking before the start up. Greg

The real reason is the Renix has no memory so it takes a couple of turns just to reset the timing and fuel mixture.
Thats also is why there's no check engine codes saved.

 

[grundan]

I know this is an old thread. I've been tinkering with my 87 4.0 and thought it was always gonna be this slow starting heep. Try this next time start yours up.
turn the key on till you hear the pump quit running, (as previously mentioned)
turn your ign. switch over and hold it for about 5 seconds in between the ON position and the start position. that little bit of dead space right between the two and then turn it over and see what happens.
When I do this to mine it fires up in about 1 second. I have no clue why it works but it does. If I go straight from on to start it will start in about 4 seconds or so.

 

[2001XJeep]

Interesting, I've owned four 4.0's over the years (currently have 3) and have never had the delayed-start problems I hear of so often. I routinely let the fuel pump build up pressure before cranking, which gives an instant light off. But even fully turning the key immediately still gives a 1 second crank time.

Now, our 2002 Mazda Millenia. It would take 3 or 4 seconds to fire, even when letting the pump build up pressure. Never did figure that one out. And that's partly why a 98 ZJ has replaced it

 

[5-90]

The real reason is the Renix has no memory so it takes a couple of turns just to reset the timing and fuel mixture.
Thats also is why there's no check engine codes saved.

Half marks. Not only does the RENIX system not "store" or "throw" codes (it's pre-OBD,) but there's also the fact that the ECU has to receive a valid crankshaft position sensor signal (always,) and it wants a valid SYNC sensor signal (which it can work without,) before it will send triggers to the ignition and fuel injection.

I'm wanting to recall that it wants to read 300RPM at the crank sensor as well, but that may be apocrypha - I'm not sure where I picked that up.

The system can work without a valid SYNC signal - it will "guess" until it hits #1TDC compression/fire, and go forward from there. However, it will take a noticeably much longer time starting (don't believe me - disconnect the 3-wire terminal coming from the distributor and try it!) and you will know something is wrong.

I've noted that keeping the battery charged up and staying on top of the starter helps somewhat. I've also got reports from the field that replacing battery cables (which are probably old anyhow, and not as well-sealed or as large as they really should be) also helps - this from people who have bought the cables I make. I don't think of that as apocrypha - I've noticed a slight decrease in start/crank times and somewhat faster cranking when I changed my own cables a few years ago - and I haven't lost either advantage yet (but I'm cranky, and tend to overbuild things.) Replacing/upgrading the cables will allow the start motor to draw full requested current, and that allows it to generate more torque, and therefore crank the engine faster (and get up to speed faster.)

I'm still inclined to try the check valve - I've just got to find one I can work with. Max FI pressure for the 4.0 is 39psi, so it's not that high (but higher than the 4-7psi used in carburettor systems, so it's still something to think about.) I haven't tried it yet - but I will give a report from the field when I finally do. Just don't hold your breath - most people do not look well with hypoxia...

5-90

 

[wilcharl]

Hypoxia can make you so happy though... I like to think of the renix start delay as a good thing.. Someone brought up the fact that it allows oil to start circulating before the heat gets turned on... I will buy that sounds good to me

 

[BillyBob]

One other thing to consider is that when you turn the key to the "start" position, not only does the fuel pump need to get a head of steam to push the fuel up to the fuel rail, but the starter is drawing quite a bit of current to get the engine to spin over, so the fuel pump will have a harder time at this point as opposed to when the engine is running. I haven't looked at the wiring schematic, but if the fuel pump resisitor was removed from the circuit during the "start" mode this could improve starting the engine slightly as it would see a little more voltage.

 

[5-90]

The fuel pump resistor was added for the 1988 model year, and was offered as a "voluntary recall" for 1987 due to fuel pump noise (I don't see why - a little noise is a good thing. Where are our priorities?)

You can baypass the ballast safely - my 1987 doesn't even have one, and I pulled out the resistor in my second 1989. It makes the fuel pump just a little louder (there's a discernable "hum" when you bypass the resistor,) but it doesn't cause any trouble, and eliminated a potential point of failure.

It's mainly there because people have some silly idea that a Jeep should make about the same amount of noise as a Lexus (I don't know where that idea came from... It's a truck, more than that, it's a Jeep! Dammit, it's supposed to be noisy!)

5-90

 

[Ron Hyslop]

The Renix xj's require two pulses from the CPS before ignition.

 

[grundan]

FWIW I did replace all my cables as well as the starter cable with welding cable w/copper ends soldered on. new battery as well.

 

[scca28]

The fuel pump resistor was added for the 1988 model year, and was offered as a "voluntary recall" for 1987 due to fuel pump noise (I don't see why - a little noise is a good thing. Where are our priorities?)

You can baypass the ballast safely - my 1987 doesn't even have one, and I pulled out the resistor in my second 1989. It makes the fuel pump just a little louder (there's a discernable "hum" when you bypass the resistor,) but it doesn't cause any trouble, and eliminated a potential point of failure.

5-90

I had thought about bypassing my ballast also. However, looking at the schematic in Haynes, it looks like the output from the ballast also goes to the ignition module. Many years ago, I had a '76 Audi Fox who's PO had bypassed the ballast to the coil, and it took me 3 coils before I figured out why they were burning out. Also, if you look at the schematic, it looks like the ballast is temporarally bypassed when the starter relay is in the start position. So, our fuel pumps are getting a full jolt at "start" anyway before the ballast takes over in "run". I haven't had a chance to check yet, but does anyone know what the difference in voltage is at the pump with and without the ballast?
'88 Laredo 4.0 Auto

 

[5-90]

Make sure your starter mounting pad is also clean - I've been kicking around experimenting with a "starter ground" cable, probably mounted to one of the mounting bolts. Haven't evaluated the utility of such a thing yet - but if you're got a dirty mounting pad (or an oil leak above or forward of the starter,) you'll contaminate the ground path for the start motor and that will also create cranking trouble (don't ask how I know, but it's fixed now!)

FWIW, I've long since installed a rocker to handle the START circuit from the keyswitch (partly for additional confusion, and partly because I had the parts handy when the ignition switch was going out...) I therefore turn the key ON, and then have to hit the rocker with my other hand. The second or so between the two seems to be enough time to let the fuel pressure build up, and it will kick right over in about two turns.

Granted, they're two fairly slow turns (compared to engines with shorter crankshafts...) but that's probably got more to do with inertia at the forward cylinders than anything else. The longer you make an engine, the longer it takes to get up to cranking speed - I had to work on a straight-eight years ago that took about half again as long to start as my RENIX...

5-90

 

[87manche]

you're correct on those points, but the ballast resistor was solely a NVH thing. My 87 doesn't have one and it does hum, and it's got the OEM coil after 250K miles.
The fuel pump does get 12V when in the start position.
The ballast resistor drops it to about 9V if I remember, but that may be wrong.