Running on 4 cyls, strange tack readings...

[Arjun]

Hmmm. Starting about 2 months ago my 89 SHO developed an intermittent low-power problem, whereby it'd be chugging on only 4 cylinders. Pulled the codes, they indicated one of the coils was bad. I checked the wires running into the coil pack, and all of them had a power signal, so I replaced the coil pack.

No go-- the problem remained. Some days I have full power, some days I've got 4 cylinders. Some days it jumps between normal and bad. More recently it's been staying bad longer, and it's at the point where I really need to do something about it-- earlier today I contemplated buying a *different* car!

Here're the symptoms:
-Exhaust note changes, tach instantly drops to 2/3 of what it should be reading, and I'm on 4 cyls. Two cyls, both fed off of the same coil pack line, aren't getting any power to their spark plugs.
-Power comes back at random, when it does the tach (usually) *instantly* jumps back up to what it should be reading, and the engine smoothes out and acts as it should.
-One time the power came back but my tach decided it was going to go nuts-- for a while, it told me that while accelerating, I was doing a constant 2000 RPM. Then it told me I was at 200 RPM even though I was in-gear and cruising around 2500 RPM. The next day I was on low power and the tach was at 2/3 of what it should be.

Recently, I finally had some time to spare once again, so I pulled out my multimeter, confirmed that there was a signal on each of the 4 wires going into the coil pack, so I pulled out my dremel and zipped apart that connector and rewired it (I thought perhaps there was a bad connection inside the connector). No go, same problem.

So where I'm at right now, I'm trying to figure out what the most likely culprit is-- a faulty DIS module? A faulty sensor? Argh! (if nothing else, I suspect it's not a wiring issue, though I'm no longer certain of anything). Any suggestions?

<small>[ May 05, 2003, 06:46 AM: Message edited by: Arjun ]</small>

 

[projectSHO89]

You electrical problem WILL be one of the following:

DIS Module
Connector on DIS Module
Connector on Coil Pack
Interconnecting wiring.
PCM Module

The DIS module generates the three coil firing signals and sends them to the coil pack via the wiring harness.

The DIS module also generates the tach signal using the CID and CKP sensors and PCM inputs. Since I have never seen the internal schematic for the DIS module, I'm assuming that the tach signal requires all the coil signals to be present in order to properly generate the signal. Since your tach is loosing a portion of its signal when the 2 cylinders go out, I would strongly suspect the DIS module is at fault.

Here are a few things to check before committing to the cost of a DIS module: Verify secure grounds (all 4 screws in the DIS/manifold crossover tube, tight grounding/mounting bolts for the tube to the intake, and a secure ground cable connection from the head to the body), if the problem occurs only after the module has been in operation for a period of time, check to make sure that the DIS module was mounted using heat sink compound. Also, it has been known to happen that faulty motor mounts will allow the rear DIS connector to come into contact with the rear engine roll dampener and will cause damage to the DIS module or the connector that runs from the module to the coil. This is also a likely source of your problem if this situation occurred in the past.

You can remove the SPOUT signal (from the PCM) from the equation by removing the SPOUT connector (small gray plug adjacent to the cam sensor). This will fix your timing at 10 D BTDC and will eliminate the input from the PCM as a possibility.

Check with your local AutoZone to see if they can test your module. Otherwise, maybe you can find a local SHO owner who will allow you to "test" his/her module in your car or vice versa.

Steve

<small>[ May 05, 2003, 08:56 AM: Message edited by: projectSHO89 ]</small>

 

[SHOZ123]

My problem was the connector at the coil. The rubber weather seal had expanded and would bunch up not allowing good contact on one pin. I had the low tach reading and it ran on 4 cylinders.

 

[MOSHO92]

My problem was alot like that. Turned out to be the DIS. Is it hot where you live?

 

[Arjun]

Thanks for the help-- I checked the DIS last night, it's mounted with heatsink paste and connecting a good ground to the crossover tube provided no improvement. Also, it's not particularly warm up here in Vancouver, BC, especially not in fall/winter/spring.

I'm nearly certain that my motor mounts are in good shape, and I couldn't find any damage on the DIS or either connector that would indicate having impacted/scuffed against the roll damper at any point.

Last night I won an eBay auction on a used DIS module-- cost me $25. Should get here in a week or so, hopefully that will fix the problem, else I'll try to track it down further and I can resell the DIS that I just bought. (unfortunately, I don't know any SHO owners around here, so I have nobody to borrow known-good parts from).

 

[ilSHOu]

I had a similar problem when one of the bolts that holds the crossover tube for the intake in place was loose. Would be something quick and easy to check. Good luck anyways, Seth

 

[Arjun]

Well, good news and bad news today...

Last night my new (used) DIS module arrived in my mailbox. It was cheap, too-- $21. Today I drove out to my box to pick it up. Installed it in the shipping company's parking lot. Fired up the car, ahhhhh, the smooth hum of the Yamahammer running on all 6 cylinders! So easy to engage the clutch and start moving! Enough power to actually make my wheels spin! Aaahhhhh, just as it should be!

Or so I thought. Half an hour later, not long after I got off the highway, I pulled onto a side street and pushed the gas to go up a slight hill. chug-chug-chug. Noooo! I was down to 4 cylinders again! So I pulled off to the side of the road, pulled off both connectors to the DIS, pulled off the coil pack connector, plugged everything back in, fired the engine up, and I was still on 4 cyls. Damn.

About 5 minutes later, while sitting at a stoplight, the car jumped back to having 6 cylinders, and I got to enjoy a momentary chirp of the tires as I sped off. Ah, that's what a like about the SHO-- lots of nice smooth power.

So, as I said, good news and bad news. It worked good, but stopped working good for a short while.

For what it's worth, it's *possible* that today's hiccup after a half hour could be heat-related-- I didn't use any fresh thermal grease while mounting the new DIS (just what was left on the intake from the old DIS), and the intake was very hot to the touch when I pulled over to take a look under the hood, and when I fired the car up after that side of the road pulling-the-connectors it did switch on the radiator fan... But then again, it's strange that it had the same problem.

Could some other DIS-related sensor be a bit flaky and cause issues? Could there be some sort of other intermittent electrical problem in a connector (eg, the coil pack connector) which is causing the DIS module to fail over time? Should I give up and take the car to a mechanic? Should I rip apart my coil pack connector and make the connection more... permanent?

Serenity now!

Thanks for all the help, folks!
-Arjun

 

[Arjun]

In case anybody's still reading this topic, yesterday afternoon I gave up and decided to take the car to a mechanic. I went to a place that I've been to a couple times previously-- they're inexpensive, the guy who runs the place is nice (though he's got a thick Chinese accent), and yesterday afternoon when I went there, his garage was exceptionally busy, with his parking lot packed and all his garage bays full. Despiet all that, he told me to pull my car partway into the garage to get it out of the rain, and he ran the codes on my car (it was giving no codes this time), and asked what I'd done, and said he was pretty sure it was the DIS module. While we were there, starting and shutting down the car, the DIS went a bit intermittent again, and started jumping back to 6 cylinders. I noticed that if I tapped the DIS module (the used one that I bought) lightly, it would jump back to 6 cylinders. The mechanic said he was certain I needed a new DIS, and this time warned me against buying a used one. I asked him how much he wanted for his time, and he said $10. $10! What a great guy, I think I took up half an hour of his time while his shop was busy! No complaints from me, I'll head back to his shop the next time my car's got problems. (TEJ Auto in Vancouver, BC).

I picked up a new aftermarket DIS module for CAD$165 at a local parts place shortly after leaving the mechanic, installed it and the car (so far) has been working great.

Last night I topped up my gas tank and reset my trip odometer, today drove to work and aroudn the city, topped up at the same gas pump at the same gas station, and for 90% city driving, over a 47KM distance, my gas milgeage was a spectacular 26.5 MPG. That's a full 100% improvement over my gas mileage while I was having the DIS problems, and almost a 50% improvement over my average city gas mileage prior to the DIS problems! I wasn't driving particularly aggressively, but then again, I don't normally drive all that aggressively.

No complaints about the performance from me!