• Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

1990 4.7 stroker running rought 10 seconds after start up

A

Art101

NAXJA Forum User
Location
San Antonio, TX
ok so i've got a rock crawler project and when we did the motorswap it ran fine. then i swapped out the peugeot tranny in favor of a aw4. after swapping in the first aw4 i started it to fill it with fluid and found that the tranny was no good, but the motor ran fine. then i put in another aw4 and went to start it to fill it with trans fluid and it didnt run right at all.

Symptoms:
after start up, it runs smooth for about 10 seconds then starts sucking in a lot of air through the intake and starts misfiring and sputtering. we have tried a new MAP sensor, and a new Crank sensor... neither fixed it

Specs:
1987 xj buggy
1987 computer
1990 4.7 stroker
1991 AW4 with TCM deleted via radesigns shifter

have not tried an O2 sensor yet
i have checked all my grounds
when you rev it up it still doesnt run right, it keeps misfiring
i have cleaned the IAC


any ideas? what would cause it to start sucking it tons of air through the intake? it runs fine until it starts sucking in all that air
 
just went out there and unplugged the o2 sensor. it runs exactly the same way it did with the o2 sensor plugged in... thats leading me to think its the o2 sensor
 
Actually you just proved it is not the O2 sensor (at east for a stock 4.0 anyway).

Art101 said:
just went out there and unplugged the o2 sensor. it runs exactly the same way it did with the o2 sensor plugged in... thats leading me to think its the o2 sensor
Click to expand...
 
cruiser54 said:
O2 sensors don't make them run that badly as a rule. Test your fuel pressure.
Click to expand...

Why the fuel pressure? The "sucking air" part has me puzzled so far.

OP, is this a renix fuel feed? If so check the FPR vacuum line for leaking fuel.

What injectors did you use? Flow rating?
 
Just some wild arsed guesses, but it sounds like a CPS problem. Maybe you have some sort of resistance in the wires or connector between the CPS and the ECU. I cut off my CPS wires a few inches out side of the firewall and soldered the connector on there and bypassed most of the other junk, shortest wires possible.

Second guess would be cross fire, one or more of your cables has a high resistance. And or the distributor cap is covered in carbon dust on the inside and the spark is jumping.

Double check your initial setup, it will start and my even idle good with the spark plug cables one off on the cap or the teeth one off on the distributor drive. Cruiser has a good TDC 1 write up.

If you have an after market cam chances are you need to index the distributor. Jeep had some issues with cam timing and the initial distributor setup. Aftermarket manufactures in the beginning timed there cams to the old specs. (some may still be doing this). An 87 cam and an 88 cam are different and may be different in other years as well, the rotor can end up so far off (with the wrong cam) the ECU has trouble compensating.

It will buck and misfire if you have a fuel problem (to lean), either too little from the pump and regulator or the something in the motor management. Way too rich is often a MAP problem.

Is it possible you plugged the injectors in out of sequence?

I'm out of ideas for the moment.
 
Ecomike said:
Why the fuel pressure? The "sucking air" part has me puzzled so far.

OP, is this a renix fuel feed? If so check the FPR vacuum line for leaking fuel.

What injectors did you use? Flow rating?
Click to expand...

The sucking air part may be cylinders misfiring or not firing at all. I've gotten my firing order wrong a couple of times and remember a sucking sound. Though it didn't actually run all that bad on four or five cylinders.:)
 
An easy quick check for ignition is to take a timing light and hook it up to the individual spark plug cables instead of the coil cable. Misfires and crossfires show up quick. You can even see weak spark this way. Just point the light at something dark, like the relay block lid.
 
That is a great list!!

8Mud said:
Just some wild arsed guesses, but it sounds like a CPS problem. Maybe you have some sort of resistance in the wires or connector between the CPS and the ECU. I cut off my CPS wires a few inches out side of the firewall and soldered the connector on there and bypassed most of the other junk, shortest wires possible.

Second guess would be cross fire, one or more of your cables has a high resistance. And or the distributor cap is covered in carbon dust on the inside and the spark is jumping.

Double check your initial setup, it will start and my even idle good with the spark plug cables one off on the cap or the teeth one off on the distributor drive. Cruiser has a good TDC 1 write up.

If you have an after market cam chances are you need to index the distributor. Jeep had some issues with cam timing and the initial distributor setup. Aftermarket manufactures in the beginning timed there cams to the old specs. (some may still be doing this). An 87 cam and an 88 cam are different and may be different in other years as well, the rotor can end up so far off (with the wrong cam) the ECU has trouble compensating.

It will buck and misfire if you have a fuel problem (to lean), either too little from the pump and regulator or the something in the motor management. Way too rich is often a MAP problem.

Is it possible you plugged the injectors in out of sequence?

I'm out of ideas for the moment.
Click to expand...
 
Art101 said:
hows that? that makes no sense, i have a handful of professional mechanic friends that say the opposite


Quote:
Originally Posted by Ecomike
Actually you just proved it is not the O2 sensor (at east for a stock 4.0 anyway).
Click to expand...

Renix will ignore a 100% bad O2 sensor, or no sensor, and go to open loop mode, and just run the engine a little rich, but it will run great, with out any engine miss or back fire.

If the O2 sensor is connected and sometimes reads right like when it is hot at 2000 rpm, but reads wrong when cold, like at idle with a bad O2 sensor heater element in the sensor, it can cause a miss at idle.
 
Art101 said:
hows that? that makes no sense, i have a handful of professional mechanic friends that say the opposite


8mud, i will check and set it up again at TDC that could be a possibility.
Click to expand...

I sure don't know how they engineered (programmed) the ECU on the Renix. But cause and effect has taught me a few things bout how it likely works. On some things like the O2 sensor the default is a standard fuel curve or maybe a choice of five (or so) depending on inputs from other sensors. Kind of a limp home mode, but actually a pretty good guess that functions well even if you loose a sensor (most times).

Getting the wrong info from a sensor can be worse than no info from a sensor, the defaults (like the O2) work well on the Renix. A few exceptions, the CPS signal needs to be there, you can even drop a few pulses now and then and it will still work well, drop too many pulses and it has trouble compensating. You have to have a base line atmospheric pressure reading from the MAP, it has to have power and the vaccum line to the MAP can't be clogged. You can clog it by plugging it into the wrong hole on the side of the TB, one hole goes thorugh to vacuum the other is a pöugged blind hole. You can even get the two hole rubber plug the vacuum line plugs into bassacwards and accidentially plug your vacuum hole. Without a baseline vacuum reading the motor may not even start and if it does, it will run very poorly.

The defaults or backup or limp home mode (whatever you want to call it) on the Renix is pretty darned good. Sensor shorts or faulty, but not failed, sensors can cause you more gief than no sensor, in some configurations. I haven't tried all of the possible sensor failure configerations, but have experimented with more than a few.

The progamm IMO isn't or is rarey ever in a standard open loop, closed loop configuration. But uses a combination of sensor inputs and defaults (standard programms) in most driving situations.

Sorry you will just have to deal with the typos, my spell checker went south and my eyes are old.:)
 
Last edited:
Ecomike said:
Renix will ignore a 100% bad O2 sensor, or no sensor, and go to open loop mode, and just run the engine a little rich, but it will run great, with out any engine miss or back fire.

If the O2 sensor is connected and sometimes reads right like when it is hot at 2000 rpm, but reads wrong when cold, like at idle with a bad O2 sensor heater element in the sensor, it can cause a miss at idle.
Click to expand...

You can also get faulty info from the O2 at mid range 2200 RPM's or so and the egine wil lean out, buck and stutter. It can also cause you to loose 1500 RPM's or more at the top end. Like you said faulty info is sometimes worse than no info.
 
Oldstimers kicking in again.

What flexplate did you use? Is there a difference in Renix-OBDI?
 
You must log in or register to reply here.

Similar threads

Z
1990 renix electrical issues
Replies
3
Views
73
cruiser54
C
N
New Engine is Hard to Start
Replies
0
Views
33
NGThomas
N
S
2001 jeep Xj Runs for a few seconds
Replies
1
Views
1K
Tim_MN
Tim_MN
N
Severe stumble, misfire and no start issues
2
Replies
27
Views
4K
maxbraketorque
M
S
Torque converter causing low rpm engine lope
Replies
2
Views
2K
lawsoncl
L
Back
Top