misfire

Have a 1993 XJ, purchased in a basket, just getting time to work on it again. It has a misfire if you rev it under no load it misses and back fires through the exhaust. And under load at WOT it misses bad. Poor performance at WOT because of consistent misfire. Otherwise drives fine at part throttle and idles good.

150 psi on all cylinders, 41 psi fuel pressure, when I key off, fuel pressure does drop but not through injectors, going back through the pump. Fuel pressure does not come up very fast, when I first key on engine off to prime, the pressure barely moves, once I crank it builds pressure and starts.

If I remove each injector one at a time the idle goes down and runs a little rough, so it appears they are all working, at least at idle.

12-14 Hg of vacuum at idle, it does have a mild cam 206/212 @ .050 .460/.476 lift on a 113 LSA. No vacuum leaks I can find, but did not have flammable brake cleaner on hand to test further. Plus would think idle would suck and WOT is usually not an issue with vacuum leak.

No codes in ECM.

My next step is to put a scope on the plug wires.

Any thoughts?

For a 93, you should have 31 psi at idle, not 41. You may have a ruptured fuel pressure regulator.

birchlakeXJ said:

For a 93, you should have 31 psi at idle, not 41. You may have a ruptured fuel pressure regulator.

Click to expand...

41 with with the vacuum line off, it is in the low 30's with vacuum going to the regulator.

Started it this morning and checked the oil and there was a white foam on the end of the dipstick. So it appears it is a head gasket.

Run a compression test. If you have low readings on two adjacent cylinders, that would confirm the head gasket failure.

FWIW, the plug wires are due to be replaced every 30,000 miles along with plugs, cap and rotor.

birchlakeXJ said:

Run a compression test. If you have low readings on two adjacent cylinders, that would confirm the head gasket failure.

Click to expand...

150 psi on all cylinders

All the plugs look good, the wires are newer wires, I'll Ohm them.

I'll record a video of the misfire, and run it longer to get any moisture out of the oil and check the dipstick again. I'm not getting any white smoke. Maybe have to do a leak down test.

I am thinking maybe a broken valve spring. The motor was rebuilt, not sure how may miles are on it.

I pulled a plug to check spark and it looks like it's running rich. Yesterday before much run time they wer ok, today they black. I also tested the temperature at each cylinders primary tube on the header and they were all at about 450 degrees at idle, so no obvious difference here. They all seem to be firing the same.



I also drained the gas that was in it, and it appeared to be bad and smelled bad. It was maybe a year old.

When I purchased it the throttle body and the injectors and fuel rail were removed. Thinking that someone was starting to tear it down to work on it. The throttle body was included, I got a fuel rail and injectors from the junk yard and had them profewssionaly cleaned in a ultra sound machine while being pulsed, and then they were flow checked. The injectors are a butterscotch color and have a part number of 33007127 with a Chrysler symbol next to it, and Siemens Deka on the other side.

Spark is good, it has a MSD 8207 Blaster coil and checks out good for the specs from MSD. However it is not the coil they call out for the application. This coil has a primary spec of.455 Ohms and secondary of 4400 Ohms. MSD recommends a 8228 coil and it has 1.2 Ohms primary and 13,700 Ohms on the secondary. Also the plug wires are about 3,000 Ohms per foot.

I have an AutoXray scanner and it is stating Adapt Fuel #1 -1024 micro seconds, does this mean it is pulling out fuel to adapt for a rich condition? Never seen this terminolgy before, use to BLM on GM and long and short term fuel trim on OBDII.

Here is what I found out about Adapt Fuel #1:

ADAPTIVE FUEL ADJUSTMENT

ADDED FUEL(uS) - XXX RANGE: -1024 to +1016 uS

On 1989 and later vehicles, ADDED FUEL(uS) indicates the adaptive adjustment made by the logic module to fuel injector pulse width at idle. A negative number means that the logic module is decreasing the pulse width from its programmed value. A positive number means that the logic module is increasing the pulse width from its programmed value.

This parameter is displayed in microseconds (uS), or 1/1000-millisecond increments. A microsecond equals 0.000001 second.


So I am maxed out with the computer taking fuel out. Will be able to do more testing tomorrow, have the day off, and will have a Genisys scan tool with scope.

Plan on disconnecting the O2 to see if it runs any better, and checking the index on the rotor to cap at the ditributor. I can do a cam crank sync with the Genisys tool also.

Any other thoughts?

Index is fine on Distributor. Pulled injectors and tested for leaking and they did not leak. Put back in and started very rich right away, cold, so in open loop and closed loop it runs very rich. Was testing with scope and started it and it ran great! Took it for a spin and ran perfect, great WOT and plenty of power.

SO I was still poking around trying to figure out why, and decided to reset the adaptive fuel with the Genisys scanner. Started it back up and went right back to what it was doing before.

I am thinking it has to be a ground issue to act like this.

Will search on adding grounds....

Grounds can be the root cause of many electrical gremlins. Refreshing grounds is NEVER a bad idea, and the investment of your time in this procedure is always well worth it!

You can't tell much of anything by looking at ground connections!! You must remove, scrape, clean until shiny the cable/wire ends and whatever they bolt to. Be sure to remove all paint from any ground connections.

Start with the one on the back corner of the head, and where it attaches to the firewall, as it deteriorates over time and is an area that makes it susceptible to damage. Best to replace that woven cable with a #4 or #2 gauge cable. You can attach the one end to the intake manifold if you would like.

Next go over to the engine dipstick tube stud. Remove the nut and clean the wire ends and scrape the block until shiny at the stud. Reattach tightly.

If you are so inclined, add at least a #6 cable from the negative terminal of your battery to one of the bolts on your radiator support.

Did you pull out the 02 sensor or just disconnect it? It may be time for a new one. And if it is dumping fuel that bad your cat will get plugged in a hurry.

If you are so inclined, add at least a #6 cable from the negative terminal of your battery to one of the bolts on your radiator support.

Or, add a #6 cable from the dipstick tube stud to the 8mm stud on the passenger inner fender just below the upper shock mount.

Will go through all grounds thoroughly, and add some.
Previous owner put battery in hatch area, or at least started to, as it is not mounted properly. Ground from battery is attached to body back there.

Unpluging O2 did nothing, when it is running rich it happens in open and closed loop. No cat to worry about,, was worried about washing out the cylinders.

Regarding the foam/milkshake oil: looks bad to me.. like coolant in the oil, it is hard to tell in the picture. Do a cylinder leak down test- compression test is just a basic tool. The leak down test will show if you have sticking/burnt valves, blow by, cracked head, cracked cylinder wall, head gasket all in one test IF you do it right.. remember TDC on compression on each cylinder one cylinder at a time-


Whats the fuel pressure DURING the misfire? You could have a pressure drop off from a regulator, pump, or crappy injector not keeping pace.

Ditto on the grounds, espcially on older Xj's TPS sensor signal ground likes to degrade- I have seen some on renix's that get fixed by running an additional ground from the ground terminal on the TPS to ground.

Don't over look TPS and IAC issues as they can casue some weirdo driveabilty issues that would swear are misfires. I had a TPS on my TJ mimic a misfire and it only went out of range at WOT when I pulled it off and measured the tbody with a DVOM

But I think you have some mechanical issues regarding that oil picture. Either it's coolant or the oil is being frothed/airated by the pump. If it is airation it could be a pick-up/sump issue with it not being sealed to the pump or debris on the screen.

With the battery back there, be sure to add a ground from the engine dipstick tube stud to the stud on the passenger side inner fender!!

Fuel pressure is good during misfire. It is a new fuel pump. I will get it good and warmed up and check the dipstick for this "foam" again. Definitely a misfire and a back fire through the exhaust, with dark plugs.

I will add a ground from dipstick to fender, and clean all other grounds. Not sure if I am going to keep the battery in the back or not.

When it is running right it has a lot of power. After I am satisfied with the grounds I will reset adaptive fuel again and see if it comes back, and post back here for future searches.

Went through and added grounds and first fire up still had the misfire. Turned it off and started it a few times and the 3rd time started ran perfect. And has ran perfect ever since, every time it is started now has been good. I suspect if I disconnect the ground or battery it will start acting up again. Now I am thinking maybe a computer issue.

Here are some videos. First 2 are with misfire, and the last one is running great.

First fire up after going through grounds:

http://s96.photobucket.com/albums/l188/T4Turtle/?action=view&current=P1000623.mp4

Second fire up:

http://s96.photobucket.com/albums/l188/T4Turtle/?action=view&current=P1000624.mp4

And the last fire up, runs great, and now it runs this way with every start up:

http://s96.photobucket.com/albums/l188/T4Turtle/?action=view&current=P1000625.mp4

Also after getting up to operating temperature the foam is gone off the dipstick, must have just been condensation in the oil.