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Running Rich/Weak Spark.... or somthing else? where to look for problems

NCSUcherokee

NCSUcherokee

NAXJA Forum User
Location
North Carolina
This problem goes back a few threads, so I might be forgetting some information, but I will try to make this description as robust as possible. I wanted to start a new thread, presenting the information in a different manner, maybe that will bring up some new ideas.

Vehicle: 1990 XJ
Engine:1988 4.0

I have not been able to get this engine to run since i swapped it in. I can nurse it to life and get it to idle fairly well, but beyond that it is pretty much DOA. I'll run down the symptoms

-Applying throttle causes popping and backfiring through the intake manifold; HOWEVER, moving the TPS to full open actually makes the idle smoother, but doesn't effect off-idle performance. It was my understanding that moving the TPS by itself would cause the engine to stumble and die.(This has me stumped)

-IAC/IAS whatever you like to call it, seems to have a detrimental effect on idle. Best engine results are obtained with the IAC unplugged. almost as if it is not letting enough air in for combustion. (To much fuel/not enough air?)

-Spark plugs were replaced when the engine was installed. They were covered in powdery carbon after running for only a couple of minutes. (This is what originally lead me to think it is a rich A/F or weak spark)

-Extremely hot exhaust, to much unburnt fuel leaving the CC. This actually melted the seat belt that was above the muffler.


The plugs, wires, cap and rotor were replaced when the engine was installed. An optima battery jumped to a running vehicle, and a replacement coil made no difference

The CPS ohms 225
TPS Voltage is variable from .56 to 4.77 and was swapped against an extra, yielding no change.
The map sensor volted out of spec so it was replaced. the line is undamaged.
O2 sensor was swapped against a working unit.
The CamPS/distributor was swapped/Indexed.
Grounds were cleaned.
Timing chain is still in correct position.

and after all that, the engine will still not rev above 7 or 800 RPM

Where else can i look for problems
 
Last edited:
NCSUcherokee said:
This problem goes back a few threads, so I might be forgetting some information, but I will try to make this description as robust as possible. I wanted to start a new thread, presenting the information in a different manner, maybe that will bring up some new ideas.

Vehicle: 1990 XJ
Engine:1988 4.0

I have not been able to get this engine to run since i swapped it in. I can nurse it to life and get it to idle fairly well, but beyond that it is pretty much DOA. I'll run down the symptoms

-Applying throttle causes popping and backfiring through the intake manifold; HOWEVER, moving the TPS to full open actually makes the idle smoother, but doesn't effect off-idle performance. It was my understanding that moving the TPS by itself would cause the engine to stumble and die.(This has me stumped)

-IAC/IAS whatever you like to call it, seems to have a detrimental effect on idle. Best engine results are obtained with the IAC unplugged. almost as if it is not letting enough air in for combustion. (To much fuel/not enough air?)

-Spark plugs were replaced when the engine was installed. They were covered in powdery carbon after running for only a couple of minutes. (This is what originally lead me to think it is a rich A/F or weak spark)

-Extremely hot exhaust, to much unburnt fuel leaving the CC. This actually melted the seat belt that was above the muffler.


The plugs, wires, cap and rotor were replaced when the engine was installed. An optima battery jumped to a running vehicle, and a replacement coil made no difference

The CPS ohms 225 and was swapped against a known working unit
TPS Voltage is variable from .56 to 4.77 and was swapped against an extra, yielding no change.
The map sensor volted out of spec so it was replaced. the line is undamaged.
O2 sensor was swapped against a working unit.
The CamPS/distributor was swapped/Indexed.
Grounds were cleaned.
Timing chain is still in correct position.

and after all that, the engine will still not rev above 7 or 800 RPM

Where else can i look for problems
Click to expand...
And the fuel pressure is?
 
langer1 said:
And the fuel pressure is?
Click to expand...


steady at 37


I should note that in my original post I said i swapped out the CPS with a working unit. I tried 2 different CPSs

Even though both units were in spec, they produced different results. With one installed the engine would idle horribly but would start easy be imposible to kill. I could unplug 4 cyliders and the engine would keep running. With the other, It would idle decent, but start hard and have a tendancy to kill itself and would imediatly die if anything was unplugged.

neither produced any kind of throttle response
 
NCSUcherokee said:
...TPS Voltage is variable from .56 to 4.77 ....
Click to expand...
:dunno:

Is this being measured at the engine controller?

Could the power/ground wires for the TPS have been reversed?
 
Lou said:
:dunno:

Is this being measured at the engine controller?

Could the power/ground wires for the TPS have been reversed?
Click to expand...

the measurment is from back probing the connector, which i belive you can only plug in one way, so i dont think it is reversed
 
Re: Running Rich/Weak Spark.... or something else? where to look for problems

How did you set up the distributor? Check and see if the MAP vacuum line is plugged in the wrong hole, there are two. One is blind the other is vacuum. Double check your firing order. Ohm test the ignition coil to cap wire. Check and see if the harness that runs down the front of the motor and back to the block temp, knock sensor and O2 sensor is fried on the manifold.

Only one way to set the distributor and have it be 99% certain it´s right, is to hook up a vacuum gage, number 1, as soon as it builds a little vacuum, line up the timing mark (clean and mark the timing mark so you can see it well) it´s always best to have your last turn to the right on the balancer, when aligning the timing mark. Slide the distributor in until the the rotor lines up close to number one marked on the cap (I usually mark the side of the distributor case also) if it has the aligning fork still on the distributor, align it with this (at the clamp bolt). Did you ever look at the pin through the gear?
There was a distributor run in 87 or so that was timed wrong, don´t think it applies to you.
I've never seen a 4.0 jump timing, but what you are describing sure sounds Dodge familiar. Jumped timing chain shows up real quick with a compression test.
The last time I moved my TPS to full open, the idle went way up. I´m guessing you have a stick shift. Intial setting, if I remember right, is .08 volts. I have an auto, so these numbers are unfamiliar to me, I set the TPS to the tranny (TCU) not the motor (ECU).
Tested between the power out and the TPS ground not the vehicle ground (there is a difference).
 
Last edited:
I just finished runing a compression test with decent results

129
140
140
135
133
135

I did test the TPS with both leads on the connector the way it is outlined in the FSM. It is an auto not a stick, do the numbers still seem correct? the initial setting was much higher than spec, but having adjusted any lower caused W/O throttle voltage to drop into the 3's. It seem like it was this way for both TPSs i used.
 
NCSU, is it possible you're using a later model H.O. TPS, instead of a Renix? I understand the signal is reversed between the two. With the throttle opened, one reads increased voltage, the other reads decreased voltage on the signal wire.
 
Runnin'OnEmpty said:
NCSU, is it possible you're using a later model H.O. TPS, instead of a Renix? I understand the signal is reversed between the two. With the throttle opened, one reads increased voltage, the other reads decreased voltage on the signal wire.
Click to expand...

no, i am using the TB from the orriginal 1990 engine
 
Just a reminder. On the Renix systems the CPS on a automatic is different then one on a standard shift. But you've said your using the one off the originial engine. So that gets you back to timing. When I replaced the distributor on my 96 I had fits getting it correct and one tooth off gave your symptoms.

JoBo
 
NCSU, you have all the symptoms of severely retarded timing.

Does your number 1 plug wire attach at the 5 0'clock position on the distributor cap (when looking down at if from the side)? Perhaps the plug wires have been moved (Clockwise) one terminal over, giving the retarded condition....?

When you installed the distributor, you did confirm the crankshaft was on the compression stroke at number 1, right? If not, the distributor would be out of phase and the engine probably wouldn't even start; just trying to rule things out......
 
OK, I just read through all your posts, and it indeed sounds like VERY retarded timing. When you advance the throttle, the timing retards even more, causing the spitting and vomiting.....

Try this: move all the plug wires one terminal CCW on the distributor cap. This will advance the spark timing only, and leave the injector timing the same. This will make a change in how it runs/starts, and maybe tell us something. If this improves the running, but not enough, the dist might still be off a tooth or two.....
 
I found an interesting problem that i have been working on. Manifold vacuum when the engine is idling is only between 7 and 8 inHg. When the throttle is opened, vacuum imediatly falls to 0 and starts to backfire

I replaced the intake/exhaust gasket becuase it was in pretty poor shape, but it made little difference. Still looking into other causes
 
NCSUcherokee said:
I found an interesting problem that i have been working on. Manifold vacuum when the engine is idling is only between 7 and 8 inHg. When the throttle is opened, vacuum imediatly falls to 0 and starts to backfire

I replaced the intake/exhaust gasket becuase it was in pretty poor shape, but it made little difference. Still looking into other causes
Click to expand...
Well that's sure no good, You seem to have a timing problem, I would be looking at the timing chain.
 
The vacuum should be the highest when at an idle. You have a bad vacuum leak. That doesn't explain everything, but fix it first. Pull and plug the vacuum hose going to the brake booster. It is the only hole large enough to drop the vacuum that far. Check the EGR is sealed to the manifold and working. Beyond that, I say you either have a baddly burned/busted intake valve or you still have a manifold gasket problem, but given your compression numbers, it can't be a valve. Check that the throttle body is sealed to the manifold.

By the way, the firing order can screw up the vacuum as well.

A tip, spray Permatex CopperCote on the intake gasket to help it seal when installing.
 
OK, this all adds up. Very low manifold vacuum, won't rev up, backfires when throttle opened, etc, etc. These are all symptoms of VERY retarded ignitiion timing. This could be from (among others), a spun gear on the distributor, plug wires misplaced on the cap, or distributor off a tooth or two (not likely because this has been checked).

I would put a timing light on it. This will tell if it indeed has retarded ignition timing, and exactly how much. I'd guess it's a lot. With retarded timing, an engine will easily start, but won't rev, and will have extremely low manifold vacuum.....
 
Print this out and use it. Check your vacuum with the coil unpluged and see what you get then.
If the cranking speed is steady (about 200 rpm) and vacuum also is steady (around 5 inches), the engine most likely is in good mechanical condition. If rpm and vacuum are uneven, the cylinders aren't pumping equally. The engine probably has leakage past the valves, rings or head gasket. If the vacuum reading is pretty steady but cranking speed is not, you're probably looking at a damaged flywheel ring gear or starter. If the cranking speed is normal or high but vacuum is low and slightly uneven, the engine probably has low compression or retarded valve timing. A jumped timing chain or belt is a common cause here.
http://www.findarticles.com/p/articles/mi_qa3828/is_200108/ai_n8978956
 
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