Tach Operating Eratically...Then Car Backfires

Ok…I was out in the 308 a couple of weeks back and 8-10 minutes into the drive, after pulling away from a stop sign, the tach began to operate erratically such that very light taps of the throttle would result in the tach jumping from 1K to 7K almost instantly…then the car would backfire.

Last October, the tach did the exact same thing and I replaced a faulty tach flywheel sensor. This fixed the problem then.

This time, I ohm’d each of the flywheel sensors and everything checked-out OK at 700-Ohms. I checked a brand-new one and it registered 700-Ohms. I checked and cleaned all of the connections, I put everything back together and now the car idles at 2K for the first ~5 minutes after start-up. The tach still jumps from 1-2K to 7K instantly and still backfires.

If one of the ECUs or coils were the issue, either the front-bank or rear-bank would run rough or erratically which they do not. Like I said, the car runs smooth…until you touch the throttle. As I mentioned earlier…the car now idles at 2K for the first 5-8 minutes of start-up.

What I don’t know is where to check next? Is there another sensor for the tach that could be faulty?

This one’s got me perplexed. Appreciate everyone's help!

 

EDIT: While the tach is showing an idle of 2K...I believe the car is really idling at 1K so, the 2K idle reading is incorrect.

 

Spiderseeker...appreciate your comments. All of the symptoms: 2K idle, jump to 7K when I tap the throttle, and flywheel sensor Ohm readings I addressed are when the car is cold.

Can you guide me to the location of the throttle position sensor? Do you know if there is a 12V reading I can check for?

I have been reading about the AAV. What should happen when you pinch the AAV hose when its cold?

I've also been told a fauly alternator may cause these symptoms...true? I had the alternator rebuilt back in Feb/March 2006.

Again...appreciate your help.

 

Verell...

The front and rear sensors are brand new and checked OK at 700-Ohms.
I checked the tach flywheel sensor which I replaced last October and it checks OK at 700-Ohms.

If the TPS is bad, would it cause the problems described? With the car turned off, should there be a 12V supply to the TPS?

What would be easiest way to check DC to the fuse panel? I did upgrade the fuse blocks to Birdmans about 1-2 months ago but I can't believe that is related.

 

FYI...just checked my car (Euro) and there is no TPS. Hmmm....

Would a bad ground for the ECUs be a source?

 

Yes...the euro's have the manifold pressure feedback hose, in fact, I thought I remember a hose going to both ECUs.

And no, I have not done the ECU ground mod. I did notice there was a ground wire in the "box" where ECUs are mounted. I've been reading that I really need to make individual ground wires from each ECU and ground them to this bolt as well.

 

I read an old post about Paul Sloans ground strap so I'll prep one and mount it to the coils.

I also read in one of his posts that "The tach runs soley off the front bank ECU (cyl 5-8, under rear window) thus an iratic tach is a sign of problems on that bank." I should probably pull the ECUs (again) and make sure everything is hooked up OK and sound. Is the 5-8 sensor the "shared sensor"?

Thank you Verell, thank you Steve...

Like I mentioned previously, the front and rear bank sensors are brand new. The tach sensor (rear most sensor) was replaced last October.

I'll go run some experiments and report back my findings...

 

Carl, you might have to guide me here, electrician / electrical tester I'm not.

By the way, I swapped the ECUs and added a 10-guage ground running from the ECU to a bold that holds the coil bracket. Neither made any difference.

Damn this is frustrating!!!

 

Carl...yes that helps!

I'll look through all of my 308 documentation to see if I can find the pinout for that connector which is on the inner part of the flying butress.

The part of this problem that continues to be puzzling is the car is idling at 1K (I know what 1K sounds like), however, the tach is reading 2K. I know if I can fix this problem, the backfiring will be solved.

Thanks again for your help.

 

Well...I haven't probed each of the pins on the drivers-side test plug, but I did swap the ECUs and added Paul Sloan's ground wire. Neither made a difference.

Just for clarification...the car idles at 1K (I know what the idle should sound like), however, the tach reads 2K. E.G., the tach is not displaying the correct idle. Not sure if that was clear from an earlier post.

 

Verell, what is really wierd is that at idle, the only sign that something is wrong is you look at the tach in that it reads 2K. The cars physical idle is normal. Then when you tap the throttle, it jumps to 7K and most of the time wants to backfire. I'm going to pull and clean every ground, connector, harness, plug wire, distributor wire...everything. I know all too well that a dirty connection or misbehaving ground makes these cars do very wierd things. Appreciate all of your input.

 

Tommy...did youget your problem figured out? Bad wires? Bad coil?

I finally got my problem solved.

1st, I noticed that the serial connector going to the ECUs didn't seem to be plugged in too tight. Even when I really made sure it was tight, the connector just seemed too loose. So, I found long (18" long) zip-ties and zip-tied the connector to the ECN.

2nd, I added Paul's extra ground with good 10 guage wire.

3rd, while I was in there, I replaced the vacuum line to the intake plenum.

One or more of these solved the 2K idle problem.

Next, just for grins, I ordered a new flywheel sensor for the tach, the one in the 2 o'clock position since the 8-month old sensor was reading a little low. I made sure everything was tight.

I've started her several times over the past couple of days and each time she runs fine. I want to take her out so bad, but here in the panhandle, it supposed to be raining for the next 6 days...Damn!!!

Oh well, this will be me some cleaning time to get her all polished up!

Tommy, let me know how your problem is coming along.