Spark Timing on a Renix 4.0

[Black1990jeep]

on my 19904 liter Renix motor, I have hunting idle, long cranking times before she starts, and at idle she smells of fuel out the tailpipe i.e. rich. But she does seem ok when on the highway, normal power and acceleration. This started happening during my last road trip, prior to that she just passed smog test with flying colors.

Engine is a factory rebuild with only about 50K miles on it.

I read manifold vacuum ad get 5 to 10 inches which varies slowly between these two values as the idle speed wanders. thus my vacuum is low (engine warmed up,throttle closed) possible causes of low manifold vacuum are.....

1..Intake leaks....., but I performed a fairly exhaustive smoke test, and NO leaks found, prior to that I tried the flammable spray method, but with a hunting idle, that diagnostic method was not effective

2.. Retarded spark timing...

3.. Bad valve timing...

4.. Bad compression... since the vacuum reading is consistent, without rapid changes, unless all six cylinders are low compresion, this is not likely my problem, hard to imagine all six sets of rings or valve started leaking all of a sudden, no excessive blow by out the oil filler.

So now I am down to spark ,or valve timing right?

valve timing issue would be a jumped chain, but with low miles on motor that is not expected
So my next test will be to check timing

But what should the advance at idle warmed up be?. Heck I can barely see the timing marks. unlike my usual cars with vacum advance that are set to the timing mark , my jeep does the advance by the magic of the ecu, thus I dont have a specification on how to check timing.

also my dizzie clamp is correctly tightened, it is not loose and moving around, Rotor and cap are good and fairly new as are the plugwires and plugs.

So what may cause the spark timing to be wrong? and how is timing checked, what amount of advance at idle, and what is max advance at what speed? I want to check this with my timing strobe.

thanks!

 

[Black1990jeep]

correction .....1990 Renix motor, not 19904!

 

[CJR]

I'd suggest you do a compression on all cylinders. Also, the long cranking starts could mean you lost the fuel pump check valve in the tank. An inexpensive external check valve is the easy fix for that.

Best regards,

CJR

 

[old_man]

A drop that far in vacuum means you have a fairly large leak. First off I would clamp off the vacuum brake booster and see if things change.

Is the vacuum fairly smooth/not oscillating fast?

 

[Black1990jeep]

Vacuum is smooth only varies as idle slowly hunts. No jerkybor rapid change as if it was one or two cylinders being low compression I think.

So how does one test the spark advance value at idle. What should it be? I want to get a timing light on it next. If that is good then either double check for leaks in case i missed one and then move on to compression testing

So what is the idle advance spec? What is max advance spec?

 

[uglyrenixxj]

I chased a vacuum leak at the transfer case vacuum switch longer than i like to talk about on my 89.
Idles like a satisfied woman with a belly full of in n out.

 

[lawsoncl]

Vac line fall off the fuel pressure regulator? Just a wild guess on my part since you said low vac and really rich.

 

[cruiser54]

Have you visited my website? www.cruiser54.com.

Put a wrench/socket on all intake manifold bolts.
The throttle body to MAP sensor hose/tubing can cause these issues.

 

[Rob Mayercik]

What about the long vac line to the reservoir blimp in the front bumper?

 

[8Mud]

Have you visited my website? www.cruiser54.com.

Put a wrench/socket on all intake manifold bolts.
The throttle body to MAP sensor hose/tubing can cause these issues.

Running rich at idle says to me the same as cruiser suggested, MAP vacuum line. That line can wear through when rubbing on a wire over the years, the hole or crack can be tiny. That line is brittle be careful.

Could be a vacuum leak, when in doubt I retorqued the intake/exhaust manifold bolts. They have a habit of loosening off. The end studs have a habit of snapping off, be careful there, I usually just leave them be as they don't back out as the other bolts do. Blow through the rear valve cover vent line, which is a part of your idle air.

Another quick check for hunting idle is the TPS, specifically the TPS ground. That ground travels through many crimp connectors that are notorious for being flaky. Not a bad idea to check your TPS setup after ohm testing the TPS ground.

It may be a combination of things, which was the case when mine was hunting at idle. Does it hunt both hot and cold?

Hard starts are pretty common with Renix. I got in the habit of turning the key to run, back to off, and then to start, which double primes the fuel rail.

Timing would be pretty low on my list of possible issues.

Does it miss at all at idle?

 

[Black1990jeep]

Occasional miss at idle sometimes, idle still hunts, dropping so low as to stall once and while, very rich smelling.

1)...I just checked idle timing with stobe, cold motor, idle hunting at higher idle, near 800 rpm the timing is 16 degrees, when idle drops low, say a few 500 rpm, the timing goes to 4 degrees. either drifting timing effects speed, or speed is effecting the timing here, which is it? the low to high idle speed drift happens over 8 to ten seconds, back and forth, hunting. Repeated this with engine warmed up more, no longer cold, same behavior)

2).... Then I created a vacuum leak on the manifold at the front vacuum fitting that supply vacuum to the EGR. I removed the approx. 3/16 th inch hard plastic tube from this manifold fitting, and the idle speed stabilizes, no more hunting, idles at about 700 rpm now, and timing is about 14 to 16 degrees. As soon as i block (sealed with my thumb) the manifold vacuum fitting I removed the hose from, thus blocking my induced vacuum leak, the idle drops very low, and motor stalled.

That was interesting, ideas? I give it a good sized unmetered air leak, and she runs better, no hunting, stable timing mark. This maybe pointing to a rich running issue due to a fault in fuel delivery rather than an ignition issue, at least that is what I is reasoning from this above induced leak test, Does that sound right to you all?

My O2 send is brand new by the way, installed about 500 miles ago when I passed smog.

3).... I carefully re-examined the hose and connectors for hose the the manifold pressure sensor just now, no defects found, and I checked it yesterday with the smoke test, a.o.k. But that was a good suggestion to double check.


4)... Still smells rich burning.

5).. One thing I failed to do was smoke test the vacuum system in the dash, that operates climate control, nor the transfer case vacuum hoses (long since disabled in place as she now has an aftermarket Atlas transfer case. I suppose I need to look under the dash, and transfer case, and along those hoses for leaks with the smoke test, or at least isolate them from the manifold and see if the problem goes away with them removed.

Suggestions on course of action is welcomed, hints, tips, past experiences etc...

At anyrate, my induced vacuum leak test (number 2 above) points me to an injection running rich cause of this problem rather than ignition. faulty sensors, leaky injectors, or?









At idle cold, I pulled off the line that supplies vacuum to the egr from the manifold vacuum port, this is one of the approx 3/16 inch dia hard tubes. When removed the idle drops to near stalling, when

 

[Black1990jeep]

ignore the last two sentences, poor editing on my part,

thanks

 

[8Mud]

Have you cleaned the IAC it may be sticky? Almost certainly gunked up and sticky if it has never been cleaned. When you clean it don't mess with the piston. Just spray it with brake cleaner and gently brush it. Clean the TB IAC well/seat really well, no need to be careful I use an old used gun bore brush. The IAC sticking can also cause hard starts. Be careful of the O rings.

The IAC may not be your issue but something you can eliminate as an issue fairly easily. The only hard part is removing the IAC mounting screws if they have never been removed before. Don't snap them off. Soak them with PB Blaster or something similar. Tap the end of the torx bit on an extension with a hammer to jar the IAC screws a bit if they have ever been removed before. A dab of grease on the threads when reassembling.

The rear valve cover vent helps add a little idle air along with the crankcase gases when the throttle is closed. Throttle open the front vent does that job. This tube plugged up seems to be a partial cause of low idle or so many have stated.

Is it possible your EGR valve is stuck open? The usual sign is it stalls at low RPM. I don't remember it causing a cyclic idle. A second way for the EGR to malfunction is the EGR solenoid. The EGR solenoid is normally open and closes (energizes) at low RPM's to close (stop vacuum to) the EGR. But like I said a stuck open EGR usually causes stalling at lower RPM doesn't sound like your issue.

Have you ohm checked the TPS ground yet, that was the main culprit when mine would sometimes idle erratically.

 

[8Mud]

Cruisers checklist covers most all the foibles of the old Renix. It is really complete and helps eliminate many issues. When I got through with his list my Renix idled like a metronome.

The ECU controls the timing, 14 degrees at around 7-800 RPM sounds about right.

 

[Black1990jeep]

found two bad vacuum fittings, the egr transducer leaked when exhaust pressure was low, when exhaust pressure was high, the leak stopped. transducer bypassed for now. also found vacuum hose to valve cover had a cracked elbow, replaced. hunting seems to be resolved for now, but idle vacuum is still low, at around 10 inches at 800 rpm, at 2000 rpm vacuum is about 17 inches. very steady

Did swap out manifold pressure sensor with spare, no change noted.

confirmed that accessory vacuum can and associated hoses are leak tight.

not smelling as rich now, but I may have become acclimated to that smell.

so still vacuum is low, but hunting has all but stopped, idle a bit rough, smells better.

need to do more

thanks

 

[my2monkeys]

The transducer is suppose to bleed off vacuum when low back pressure is present, thus not letting egr to function. I would hook it back up.
Check the fuel pressure regulator, pull off vac line and see if it smells like fuel indicating an internal leak that sucks fuel into intake manifold.
Do you still have the original 2 piece injectors? They are prone to leaks at the body segments.

 

[Black1990jeep]

more data...

Viewed spark wires in dark, used wood stick to push the wire around at each plug, found arcing on one of them, then found ceramic insulator was cracked on that plug.

all six plugs removed, only one was cracked, although all looked a bit sooty on half the center electrode.

compression check, wide open throttle, engine warmed up. all values 142 to 155 psi, thus compression test is good.

Stop for the night, new plugs to be installed tomorrow. will update.

never had a busted spark plug like that before.

thanks

 

[Black1990jeep]

The transducer is suppose to bleed off vacuum when low back pressure is present, thus not letting egr to function. I would hook it back up.
Check the fuel pressure regulator, pull off vac line and see if it smells like fuel indicating an internal leak that sucks fuel into intake manifold.
Do you still have the original 2 piece injectors? They are prone to leaks at the body segments.

I think these are original type injectors I have only had this jeep for two years. I dont think prior owner changed them, but I dont know what stock ones should look like

ok, so the EGR transducer is supposed to bleed off vacuum, weird. I figured it would simply seal off the vacuum rather than create a leak. thanks for setting me straight on that!

I wiggled each injector, no fuel leakage around them.

Fuel regulator seems ok, no fuel leak from its diaphragm. This regulator is about a year old, replaced when other one leaked .

Thank You!!!

 

[AJPulley]

What age are the injectors? What is the fuel rail pressure at idle when engine is cold and at temperature? Is the EGR solenoid energized or not at idle (ECU provides the ground for it)?

 

[AJPulley]

Disregard my first question. I see you thing they are original.