Jump to content

Recommended Posts

I have a 95 YJ 4 litre let's running extra rich. It will foul out the spark plugs with carbon fouling within a few minutes.

 

Let me start at the beginning, I bought this Jeep running terrible.  I was told it had a dead battery while jump starting they hooked up the cables backwards. I thought this was probably true due to the fact that both alternator fuses were blown.  I drove it nine miles home and went through over 4 gallons of gas. Most of that ran out the tailpipe or was dumped into the crankcase.  The number 5 and 6 injectors we're running fuel anytime the key was in the run position.  I diagnose that problem to a burnt PCM.

 

The PCM was sent out to a repair facility and upon return was installed and no start. the light on the dashboard would not even light up. I was told by the repair facility that that was their fault send it back and I would get a replacement.  Upon installing the replacement PCM, the engine would start but still running very rich. The injectors no longer run when they're not supposed to.  I started doing test procedures according to the Chrysler service manual.  Cylinder leak down test shows all cylinders between 12 and 10% leakage.  All cylinders 125 to 130 psi compression.

 

Cam position sensor checks out ok crank sensor okay distributor cap rotor wires and plugs all new. timing ok per the book. ignition coil showed a low ohm reading on the primary side so I replaced it. spark is OK MAP sensor is OK idle air control solenoid was sticky so I replaced.  Sprayed carburetor cleaner all around intake and didn't show any vacuum leaks.  Manifold vacuum on my gauge reads low. About 5 inches at idle and up to 12 to 15 inches when rev up.  Pulled the o2 sensor and plugged in a pressure gauge to do back pressure. Shows 1 pound pressure. Doesn't go up when engine revved so catalytic converter doesn't seem to be clogged.

 

Running rich enough that in the three weeks I have been trying to figure this out, the new spark plugs were fouled enough to make hard starting. Since then I have installed fresh spark plugs. It will start and run but terribly rich.

 

It acts like its not timed right, but all that is controlled through the PCM.  Every check I do seems to point to the PCM still not being right.  I am now one step away from doing the 5 gallons of gas and a match procedure.  What in the heck am I missing?

Share this post


Link to post
Share on other sites

dinko...First off, congratulations on using a factory workshop manual and doing a professional approach to diagnostics.  You nailed everything I would check, including the crank and camshaft position sensors, a definite area of concern. 

 

I read your diagnostics carefully.  Your account of the PCM issues does sound accurate, you likely needed the PCM repair.  The leakdown test results are impressive and great.  The compression could be higher, but that's likely from fuel wash of the cylinders due to the over-rich condition.  Once you resolve this problem, the cylinder seal will be better and cranking compression will rise. Check cranking compression with the throttle open when cranking.

 

The trouble spot I would address is fuel pressure.  Your rail pressure could be too high, or the return to tank line on the two-rail system could be clogged or misrouted (like switched with an emissions EVAP line).  Something may be causing high fuel rail pressure.  This could be a clogged return or a pressure regulator stuck shut, which is less likely but possible.  Fuel pump pressure is quite high when compared to regulated pressure, and if there is a "wall" that pressurized fuel hits, the high pressure at the rail will cause injectors to flow rich—even extremely rich.

 

Check for a free flowing return to tank from the rail.  Check the pressure regulator.  Normal pressure at the rail is approximately 39 PSI with the vacuum line disconnected from the regulator.  Running at idle with the regulator vacuum hose connected should produce around 31 PSI.  Pump pressure can reach 75-100 PSI with a pinch test at the return line.  A kinked or obstructed return will raise rail pressure high enough to flow a large excess of fuel and create a very rich running condition.

 

I'd target the return restriction prospect.  The regulator could be defective, but that would be less likely.  Run the common rail pressure check and report back!

 

Moses

Share this post


Link to post
Share on other sites

Moses,

First of all, thanks for the quick response. It's great to have a place to go to get nolageable information.

I was so frustrated by the time I did the last post I didn't put down all the things that I have checked.

I did do the fuel pressure test. At first I did have a high pressure approximately 70 psi. I thought great I found the problem. I traced it down to a pinched hose return line on top of the fuel tank. The tank was removed the lines straightened out and now the fuel pressure is 32 psi.

I have the EVAP canister disconnected at the moment the lines plugged so as not to have a vacuum leak.

The reason I'm leaning toward a timing problem, I am thinking of a jumped tooth, is because of the carbon fouled plugs also that carbon is in the intake manifold. I would think that the intake valve is not completely shut when its trying to fire.

On a normal engine, one would be able to look at the position of the distributor compared to the timing mark on the dampener Pulley and determine if this had happened. I'm not sure how to do that on this motor.

The position of the rotor and of the metal piece in the distributor are positioned correctly according to the service manual but I am Not sure that the chain hasn't skipped a tooth. My next project is going to be remove the front cover and verify timing is in the correct spot.

Mike

Share this post


Link to post
Share on other sites

dinko...Late valve timing is always a possibility, and this would also explain why the cylinder seal is fine but cranking compression is not as impressive.  In the case of valve timing being only off a tooth, or more likely a sloppy and worn chain, your cranking compression would not be as high as you would expect for the good cylinder seal indicated in the leakdown test.

 

If this engine has higher mileage, the timing chain could be loose.  "Jumping a tooth" is extreme and would be quite noticeable as poor low speed performance but okay top end. Your idle vacuum of 5 in/hg with 12-15 in/hg as the engine speed increases could be a cause for late valve timing if there is no vacuum leak or dramatic spark timing discrepancy.  Late spark timing will also drop idle vacuum, but this is less likely with a PCM and fixed crankshaft position sensor.

 

Before removing the timing cover and verifying the timing chain condition and timing marks, try this approach, my time-honored fast check for timing chain wear:   http://forums.4wdmechanix.com/topic/174-quick-test-for-engine-timing-chain-wear/?hl=%2Btiming+%2Bchain.  If the timing cover does not look like it's been removed recently (possibly for a new timing chain, which could have been installed a tooth off), you're looking for a loose chain and late valve timing.

 

How many miles does this engine have on it?

 

Moses

Share this post


Link to post
Share on other sites

The odo says 205k. 90k of which were supposed to be tow miles. (not sure I believe that).

Engine is dirty enough for me to think that no work has been done for awhile.

I will look at the link.

Thanks

Share this post


Link to post
Share on other sites

I did the timing chain slack test and had 8 degrees deflection. Meaning with the timing mark at 0 I backed it up to 8 degrees before top dead centre when the rotor started to move.

Share this post


Link to post
Share on other sites

dinko...This chain slack is not extreme, though we're still not sure whether the sprockets and chain align correctly.  The 8 crankshaft degrees width includes slack between the chain and sprockets plus distributor drive gear backlash with the camshaft.  A "jumped tooth" is virtually impossible with this amount of chain and sprocket slack, so is human error during a timing chain replacement—the timing cover hasn't been removed.  I'd set aside the valve timing prospect.  Save yourself the trouble of removing the timing cover, at least for now.

 

Have you watched ignition timing from idle to 2500 rpm?  (Stay away from the engine's fan during this kind of spark timing test and use a timing light with built-in advance adjustment.)  Does spark timing advance appear "normal" from idle up?  You did check the distributor indexing, right?  There are only four causes for low vacuum readings at the manifold:  leaks, late valve timing or late spark timing and, in MPI/EFI systems, bad feedback from sensors like the coolant temperature sensor or TPS.  

 

You replaced spark cables, cap and rotor.  Did you test the O2 sensor, which is always suspect, as failure would throw the PCM/engine into WOT limp home mode, which could be what you're experiencing...I'd test the oxygen sensor, the coolant temp sensor and the Throttle Position Sensor (TPS).  These fail from long duty cycling and are easy to test or replace.  To pinpoint trouble, test each device before replacing anything.  If voltage is off on the TPS, there is confusion between the throttle valve opening and fuel flow.  The O2 sensor has been exposed to an over-rich mixture for a long time and could simply be overwhelmed with carbon build-up.  Any trouble here would throw the PCM into WOT/limp home mode.

 

Note: Your system is OBD-I that requires a device like the DRB-III scan tool for diagnostics.  A DRB-III or equivalent scan tool can "cycle" devices and test them.  In this case, however, you can perform these tests readily using a simple volt-ohmmeter.  Follow steps and compare voltage requirements described in your workshop manual.

 

The target is that very low 5 in/hg vacuum at an idle.  If the O2 sensor, TPS and coolant temp sensor test okay, consider an excessive vacuum leak that the engine is trying to offset.  Check for a vacuum loss at the front axle disconnect motor, the transfer case vacuum hoses, the EVAP and so forth.  To be quicker, try disconnecting and capping off all vacuum fittings at the manifold.  Do not overlook the brake booster check valve or a booster diaphragm leak.  (You can check for a booster or check valve defect with a simple vacuum gauge/hand pump.)  Check the manifold vacuum at idle with all hoses disconnected and fittings capped off.

 

Moses

Share this post


Link to post
Share on other sites

Moses, I have the entire manifold plugged except for the MAP sensor had that plug that one time also no difference in the manifold vacuum. I got carried away on another project of fixing my exhaust leak and at the same time replaced my o2 sensor. The TPS ohms test ok. The coolant temp sensor at room temperature which is approximately 80 degrees reads open. No resistance at all. May be a problem?

I did get into the timing chain and it is position exactly lined up the marks on top dead center number 1 compression. Pulled the distributor out and double check the indexing on it. The only weird thing is that when the distributor is installed according to the service manual, the rotor points almost halfway between the distributor caps number 1 and number 5 terminals. This equates to about 45 degrees before top dead centre when it is in line with the number one post of the distributor. the rotor points almost halfway between the distributor caps number 1 and number 5 terminals. I was contemplating pulling the distributor out and rotating it back one tooth on the gears to try and get that closer to the number one post at top dead center. But in doing so then the cam position rotor will not be in the correct spot according to the indexing holes in it.

The folks that sold me the ECM are supposed to be sending me another one this week.

Share this post


Link to post
Share on other sites

Oh. Almost forgot to mention that when I hook up my timing light, no advanced options, I cannot see the timing mark.

Share this post


Link to post
Share on other sites

Dinko...On the distributor indexing, set precisely to factory guideline, including the cam position sensor.  Do you have the FSM instructions handy?  Did you follow this closely?  Keeping in mind that there is no centrifugal advance mechanism, there must be allowance for the electronic advance.  You can find that the rotor does not align "properly".  This allows for spark timing advance, although you're off quite a bit here.  Do you need the steps for installing/indexing the distributor?

 

The timing mark is only visible if you follow the factory procedure for checking base timing.  Check the manual, if you don't have that procedure, I can provide it.  Without the right wiring hookups during the test, timing will be way off the mark when the engine is running without a load.  This is not like conventional timing, the advance is electronic and governed by load conditions and emissions requirements.  Idle advance degrees can be considerable.

 

Do you need the resistance specs for the coolant temp sensor (CTS)?  Can provide...

 

Moses

Share this post


Link to post
Share on other sites

Yes I have a factory service manual. The distributor is in exactly according to their specs. The haven't checked the timing as per the instructions yet. I went ahead and purchased a new coolant temperature sensor. Did not make any difference though.

Share this post


Link to post
Share on other sites

Moses,

I was looking up some of your other posts trying to find out information on your test light setup. I ran across a guy that said when he plugged his intake it still ran. I tried it and it died immediately. No vacuum leaks.

I unplugged numbers 5 and 6 injectors and the idle smoothed out (not rich) and started running better.

I unplugged each of the other injectors one at a time (5 and 6 still unplugged) and a noticeable change in the idle.

Also, with 5 and 6 unplugged, the vacuum is up to 12 to 15 inches!

I definitely have some more diagnostic work to do.

Share this post


Link to post
Share on other sites

This really sounds like too much fuel flow at the injectors.  Check your fuel rail pressure while testing in the manner you just shared.  Acts like high pressure or clogged/obstructed return-to-tank.  Let's look at fuel rail pressures while this is going on.  A Harbor Freight fuel pressure tester would do here; obviously, there are better devices in the market if you want to invest.

 

Wiring crossed at the injectors is unlikely, but a defective PCM could be opening and closing injectors incorrectly...Test the camshaft and crankshaft position sensors if you haven't done so already...Just sayin'....

 

Thanks for keeping us informed and being so thorough.  This is a breakthrough on the manifold vacuum, anyway.

 

Moses

Share this post


Link to post
Share on other sites

Well, #5and6 injectors not getting any signal from PCM. Fuel pressure perfect. All sensors working fine. Still coughing and sputtering.

Replacement PCM came in the mail today.

Installed it and now the engine purrs like a kitten.

Hated to have to take all the time to test everything, but now I know that everything is all good.

Haven't heard much good about the computer rebuild industry but hopefully the third time is a charm.

Thank you all the advice and now that I own a Jeep I'm sure I will talk to you again.

Mike

Share this post


Link to post
Share on other sites

This did point to the PCM.  You did a great diagnostics approach and avoided wasting energy and money in the process...Good job! 

 

Moses

Share this post


Link to post
Share on other sites

×
×
  • Create New...