1992 LS400

I was doing routine maintenance by replacing the spark plug wires. Also started to replace the fuel filter but decided to wait until the tank was near empty after loosening one of the fuel lines.

Symptoms: Starts fine, idles slightly rough but livable, the problem is when accelerating. The engine gets extremely rough, knocks if you push it too hard, when in gear struggles and sputters to get to 25 mph. No power at all, but does not die. Can smell unspent gas in the exhaust after running for 10-15 seconds.

Work So Far: Ohmed out the spark plug wires (all 500 to 1.8k), ohmed out the coils (both primaries measure 0.9 ohms and secondaries are 12k-13k ohms), coil primary spec is 0.41 - 0.50 ohms but I figure the average RadioShack meter surely is not that accurate, cycled the fuel pump a few times and started the engine a few times checking for leaks around the fuel filter, checked to make sure the intake plenum and TB was still clean from the cleaning 2 mo's back, verified all plug wires are going to the correct locations, disconnected the exhaust after the cats and verified sufficient exhaust flow to rule out cat blockage or exhaust blockage and easily blew any dust out from the garage floor under the car-no significant improvement in off-idle performance so no blockage in the cat-back system. Removed the intake filter with no significant change to off-idle performance. Cleaned the sparkplugs (3 mo old). Inspected all vacuum lines for any breaks, tears or pulled off hoses, Still wondering if it is the fuel filter but the unspent gas smelling exhaust leads me to believe that it is getting more than enough fuel. Thoughts?

 

This is EXACTLY the same symptoms that my 92 displayed when my coils went bad. Replaced them with OEM and problem solved.

 

Were the bad coils measuring out of spec? Primary 0.3-0.6 ohms / Secondary 9-15k ohms?

 

Just a wild thought if it wasn't the coils. Is there any chance at all some contaminated gas screwed up some injectors ??? You said you smelled unspent gas so.....

 

I honestly did not measure them for spec. My thought was it was most likely an original part and my car,like yours, is 17 years old. It was time to replace them, since it was on my "to do" list of things to update/replace. I just went ahead and replaced both of them. With the help of some members here with my symptoms, we narrowed it down to the coils and that WAS my problem. Like I stated b4, my 92 was doing the EXACT same thing yours is doing now when mine went bad.

Cruiter,, Bad gas was my initial thought too, but when I had the problem, I was almost on an empty tank, and had been riding around of the tank of gas for quite some time. I normally stick with the same gas stations when I fill. I went ahead and filled up with a ful tank when my problem started and it did nothing to solve it.

 

Bad Gas?: I'm not ruling anything out at this point, but the tank is 3/4 full and showed none of the symptoms between last fill-up and pulling into the garage to change the wires.

Coils: I did some quick math, typical coil measurements of Primary = 0.5 and Secondary = 12k will convert 12V into 288kV, my measurements of P=0.9 and S=12.3k would only generate 164kV. A small difference in the primary measurement knocks the voltage to the plugs down significantly and possibly below the threshold of normal operation.

I will re-measure the primaries with a better meter this evening. I am going to change out the fuel filter since I already have the part. Anyone know if a way to test the fuel pump. I here it turning on when the ignition is moved to the "on" position but are there any specs for impedance across the pump motor? Has anyone seen caps, rotors or fuel pressure regulators causing anything like this? I might try disconnecting one plug wire at a time. Each should make the engine limp worse. My thinking is that the one that does not change anything when removed is not making a good connection from the rotor to the cap, the cap to the wire, the wire to the plug or the plug is bad.

 

I tested both coils using an industrial digital meter with a calibrated 0-20ohm scale.
DriverSide: Primary=0.45 / Secondary=12.7k
PassSide: Primary=0.45 / Secondary =12.8k
(side note: the hook on a paint can opener works wonders at releasing hard-to-reach electrical connectors like on the coils.)
Both coils are within spec and off the suspect list although I might still replace coils/rotors/caps if I still have a budget after all of this (good point the 17 yr old car comment). I rechecked the plug wire routing according to a manual excerpt I found online. All were correct. I replaced the fuel filter (don't wear a shirt you plan on keeping when doing this). Still not solved, but I may have an idea. While priming up the fuel pressure after the filter swap, I noticed that I could not hear the fuel pump whining. I even pulled the carpet out of the way and put my ear on the gas tank and still can't hear it. I listened to the fuel pump relay at the front passenger's side engine compartment while my wife switched the ignition on and off and not a sound or movement inside. I pulled the cover off of the fuel pump relay and it engages once the engine is running. A quick look online says that this relay gets voltage from the ECU (not a fuse to switched 12V). Suggestions other than lay out the $350 for a ECU? Possibly a sensor?

 

More web reading...I am going to pull and clean the IACV next. I doubt that it is causing the problems beyond the rough idle but the level of gunk found in the TB a few mo's back tells me the ICAV is just as clogged and may be at least contributing to the problem. I am also going to try the fuel pump controller bypass trick to see if it is faulty. Both are free and WAY easier to do than replacing the caps and rotors that no one is suspecting as the culprits at this point. What should I try after that? Does anyone have a systematic way to step through testing each of the sensors tied to the ECU. Specifically, the ones that the ECU uses to determine timing adjustments?

 

Sounds to me like the plug wire routing is still wrong.


1-8-4-3-6-5-7-2 is what I show.

2-8-c-3-5 passenger bank distributor

4-6-c-7-1 driver's bank distributor

The passenger bank of cyls are EVEN numbered, driver's are ODD.

 

The other thing to look at is if you reconnected the MAF correctly...if there are loose or missing parts in the intake tract.

 

Cleaned the IACV valve (not terribly clogged), did the fuel pump control unit bypass trick and reset the ECU. Started her up and idled a little smoother but still not out of the woods when accelerating. I removed the fuel pump bypass and and restarted. At first, it did not want to stay running but leveled out after a few minutes of calibration and had a fairly steady idle. I pulled it off of the jack stands and took it around the extended block. Never died even at a stop light, some light pinging when you floor it. Acceleration is better and can get up to 40mph (before topped out at ~25mph). It takes just about flooring it to get there but does so like a heavy car from the 70's with a smart car engine transplant (way slow acceleration). I could out run the neighbor kid on his bike (eventually) but something is still preventing normal power and acceleration. I think the ECU reset is trying to mask the problem by adjusting the timing as far as it can go, etc, but something else needs to be done to really fix it. Thoughts?

 

[guess on]
MAF sensor plug, check all connector lugs for proper latching in the harness.
[guess off]
Colin

 

Good idea, I will check it in the AM. While I am at it, how do I test the MAF unit itself?

 

Sounds like its misfiring. I know you spec'd and checked the wires, but this same exact problem was on my friend's C280 and the culprit was the wires (actually, one of the stems from the coil pack but we wouldn't have guessed it in a million years as it was producing spark). Pull your spark plugs and see if they are black with no oily residual. Then, replace the spark plugs, and with the engine on, remove the spark plug wires one by one to see if it affects the engine's performance (replace after testing each one).

Not helpful I know but it's the best we can do.

Your car not throwing a CEL?

 

unplug it...car should not run.