Back Firing Through Intake

89xj 4.0 auto. Just completed replacing timing chain and sprockets. Fuel pressure regulator at fuel rail. New plugs, wires, distributor cap, rotor button and CPS. Started like a new one yesterday and ran fine allday. Went out this morning and no start but would crank. After numerous tries and backfires it started and is purring like a kitten. Drove several miles and turned it off. Started perfect. It was 16 this morning. What have I over looked? These are the same symptoms occurring before I did any work on it. 190,000 miles. Please Help! Thanks so much got alot of excellent help comments on the the work I've done!

If this is a one time thing, don't worry about it but in general, backfiring in the intake can indicate a lean fuel air mixture.

Verifying the exact fuel pressure with a gauge would be a good idea if this happens again. Should have 31 psi at idle, then should jump to approx. 39 psi with the vacuum line to the fuel pressure regulator disconnected.

Other engine management sensors in play include the coolant temp sensor and the map sensor. Here is a link to a good article on testing all things RENIX.

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

So...how much fuel pressure do you have?

If I unplug the Idle Stepper Motor while engine is running what should happen? When I unplugged it while the engine was running it didn't miss a beat.

Idle Stepper Motor Unplugged

If I unplug mt Idle Stepper Motor also known as Idle Air Controller while my 89xj 4.0 is running what should happen? My engine continues to run perfect. Having a very had time starting when it is cold. Any suggestions?

Re: Idle Stepper Motor Unplugged

I've lost count...do you now have 3 or 4 threads started with your engine problems?
You should, in my opinion, keep one thread alive so we can better help.
Someone looking at THE IDLE STEPPER MOTOR UNPLUGGED thread won't know how many parts you've changed.

dennis461 said:

So...how much fuel pressure do you have?

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I have not checked fuel pressure. Is this a must for my symptoms. I have no guage. What do I need?

Re: Idle Stepper Motor Unplugged

Sounds like your IAC isn't doing its job.

Has "Uncle Bob" ever worked on your XJ? He is quite famous for tampering with the "idle screw" on the throttle body. The problem is there is no "idle screw"--there is, however, a "stop the steel throttle plate from slamming home and damaging the alloy throttle body walls screw"--and that is what "Uncle Bob" likes to play with.

Remove the intake tube from the TB. Using a 3/32 Allen wrench back out the aforementioned screw until the throttle plate is no longer effected--so that it is fully closed. Now, very carefully observe the throttle plate while you slowly turn the Allen screw back in--at the first sign of movement of the throttle plate STOP. You are done.

Now, might as well clean the throttle body while you have things already apart. Remove the IAC. Pay particular attention to the IAC port on the TB making sure it gets thoroughly cleaned. Clean the pintle on the end of the IAC gently--it can break. Reassemble everything. Now, adjust the TPS because undoing what "Uncle Bob" did has changed things.

Ok, now you should get an idle of 700-750 RPMs after the XJ has been driven for a full 20 minutes and is fully warmed. If AW4 equipped the idle is checked in DRIVE with the parking brake set and someone's foot on the service brake pedal. There you go--that is how it is done. The IAC is a stepper motor that responds to extend/retract commands from the ECU. The ECU uses data from a number of sensors to decide what position the IAC should be in.

Don't let "Uncle Bob" screw the pooch.

Sorry Dennis. I have been working on this xj all day. I am dirty,cold,frustrated,and broke! Just want to correct my problem. Will you help?

ken johnson said:

I have not checked fuel pressure. Is this a must for my symptoms. I have no guage. What do I need?

Click to expand...

See if one of your local chain parts stores rents a gauge.

You attach it to Schrader valve on the fuel rail.

eBay, Amazon, Harbor Freight are all sources to purchase one. You can get the ultimate cheap ones for less than $20 or get a professional quality set for around $500. As you won't use it a lot I would suggest something around $35-$50 that will give you enough accuracy and that would be likely to work when you get it.

EDIT: when you search, use "fuel injection pressure tester", you don't want a low pressure carb tester.

I unplug the Idle Stepper Motor while engine is running what should happen?
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I DON"T THINK the step motor does not close when upluged. As long as you don't place any addsional load on the engine she will idle no problem.
Turn the wheel and the exta load of the power steering pump will couse your RPM to drop or stall.

If you unplug the IAC when the motor is running, other than running the chance of frying the drivers in the ECU, you should see no change in speed.

badron said:

I unplug the Idle Stepper Motor while engine is running what should happen?
=========================================================
I DON"T THINK the step motor does not close when upluged. As long as you don't place any addsional load on the engine she will idle no problem.
Turn the wheel and the exta load of the power steering pump will couse your RPM to drop or stall.

Click to expand...

Hard starting:
do check fuel press.
Check timing this will also couse backfire as well hard start.
If the dis has to be ((((WAY))) off from were is was before replacing the timing chain inorder to start. You may have the the cam one tooth off.

badron said:

Hard starting:
do check fuel press.
Check timing this will also couse backfire as well hard start.
If the dis has to be ((((WAY))) off from were is was before replacing the timing chain inorder to start. You may have the the cam one tooth off.

Click to expand...

My timing at normal idle is 11 BTDC. Is this about right?

check firing order.
Sorry I don't remenber if the 89 uses a CPS or iif the pickup is in the dis? If it use a CPS then timing can not be ajusted by seting the dis.
With a CPS the dis can be turned but this will not change timing. But ((IF)) the dis was moved way off this can couse the wrong plug to fire or force the sparck to jump a big gap in the cap.
To check this some blocks will have a notch in it as well as the dis base. Just aline notches.
No notches. A peck-a-boo hole can be cut in an old dis cap so you can see when number one is firering.
Remove number one plug. Remove value cover. (((((INSURE))))) number 1 is fireing by watching for number ones intake value to be closing. CARFULLY palce a screwdriver in number one plug hole. You will be able to find TDC when the driver is at it's highest point. Turn engine by hand. (((((DO NOT BIND DRIVER)))))
With number one intake closing and piston at TDC the rotor should be pointing very close to the number one on the old cap. Ajust as needed.