I started out with this post in the TPI forum, but after no replies in 3 days, I decided to try the experts over here....

I've been chasing an apparently random miss on my 383 SuperRam setup, and been DIY prom-ing as well. Can't seem to get it consistent so did some more trouble shooting and here's some of the details of things I've found.

Initial driving in the morning (cooler 80* or so temps) is the best. And that's while in closed loop.

Turning on the A/C with the fan on high when temps are high (95-100 here in Dallas) is guaranteed to cause it to almost stall at idle, and causes the miss to occur very noticeably.

While datalogging with Datamaster, I noticed that the battery voltage is usually only showing 13.5 or so volts, and when the A/C is turned on that drops to about 12.7 - 12.8 and occasionally even a little bit less.

Thinking this was too low, I went by a shop and they hooked up their meter to the battery cables while the car was running to try and check the alternator output.

The interesting thing here was they were showing 13.8-14.0 volts even with the A/C turned on as well as the headlights and radio. But Datamaster was reporting that the ECM was seeing at least 1.0 volt less than they were reading on their meter (12.8-13.0v or less).

I know the ECM varies the pulse width based on the voltage it's seeing, so I'm wondering if this is a source of my miss.

Why would the ECM be seeing less voltage than is going into the system from the alternator? And how much effect would this have, if any?

If I truly have a problem with this lower ECM voltage, what should I look for? Suggestions, ideas, fixes?

Sure hope you folks over here can help me figure this one out...


- Vern

 

Does your fuel pump feed wire (to the relay) come from the same feeder circuit as your ECM? or somewhere there may be a high current draw nearby.
The wire to the fuel pump circuit could be marginally sized and causing a voltage drop at the ECM. Maybe adding a second wire in parrallel as a trial would show something.

I originally rewired mine that way and came to the conclusion quickly that it was a bad idea to connect them from the same source even though it was a 10 awg wire to the fuse block. My pump now feed directly from the battery.
Somthing to consider.
I'm not sure how "stock" your wiring is.

The stalling could be getting too much fuel if the ECM sees the lower voltage and bumps up the pulse width (and the stall saver issue from RBobs thread)
John

 

Hey Vern,

I recently picked up a craftsman engine analyzer and figured I would just check it out after reading your thread. Was kind of curious also about the readings you were getting. I had also put an auto x-ray 6000 on it a few days previous. With the engine running at idle, the voltage gage in the car was reading between 12.9 & 13.1 give or take a hair. The voltage reading on the analyzer was reading about 14.4 give or take a hair. Turned on the radio & A/C and the voltage dropped slightly but was still proportional between the gage & analyzer. My voltage reading with the scanner was just a tad lower than the voltage gage by about .2 volts. While I was at it, I also measured my RPM's coming straight off the ignition coil at the test lead. The reading there was a little over 100 RPM lower than my tachometer on the dash and roughly the same as the scanner reading. Don't know if this will help you with your problem but I thought I would pass it on.

 

Thanks for the reply and effort, Trickster.

I fully realize the in-dash gauges are sometimes "less than accurate", but I wasn't including their readings. The ECM voltage I was reading was right out of the Datamaster display and log files read via the ALDL connector as reported by the ECM. That voltage is reading at least 1v less than what a known correctly calibrated meter will read across the battery cables at the same time. Somehow I'm seeing a voltage drop between the alternator/battery and the ECM. If I read you right, your scantool connected to the ALDL shows the same voltage (within .2v) as the at-the-battery cables reading. So I shouldn't be seeing this kind of a drop. D*MN!

Guess I'll just have to get my meter out and start taking readings at various points along the harness on it's way to the ECM. Gonna be a b*tch since everything is still loom in-cased. I was just hoping for some better ideas.

Now to go dig out the wiring diagrams so I can see which connection points to the ECM should be the power and start tracing them back from there..... <sigh> gonna be a LOOOONG weekend, I'm afraid.

Once again, Thanks Trickster!


- Vern

 

There should be two voltage readins available on the scan tool: Battery volts and Fuel Pump volts.

The Battery voltage reading is the ignition feed to the ECM.

The Fuel Pump voltage reading is read after the fuel pump relay.

First check to see which reading is being displayed. Then measure the voltage between the battery + and that point. This will show the voltage drop.

Can then further narrow down where the majority of the voltage is being lost by measuring between the battery + and feeds along the way.

It could be a failing fuel pump relay or a failing ignition switch. Or, just a bad connection someplace.

OTOH, it may be normal. I don't know what typical voltage drops in the system are.

RBob.

 

One word.... grounds.

Any difference in ground potential will cause different voltages.

Make sure all grounds are clean, including the ECM grounds at the rear of the heads. Even a little grunge/oil/dirt can cause different voltage readings when checked at various points.

However the ~1V difference your seeing is more than likely normal.

I'd be more conserned with what the voltage reading is at the fuel pump connector. Even a 1 volt drop will decrease efficency. The ECM is more forgiving.

Bob

 

Actually Vern,

What I said was that the scantool connected to the ALDL was reading just a fraction lower than what the instrument gage was reading. The voltage reading for the fuel pump was about 13.6 volts. The engine analyzer connected across the battery terminals was reading about 14.4 volts at the battery from the alternator.

 

First I'd check the pins at the connections for the ECM and grounds, they may be a little dirty or loose and causing your voltage drop. Then its time for the spark chasing

 

Spark Chasing?

 

Highly technical term for chasing down shorts & electrical troubleshooting.

 

Anything more than .5 volt is too much, and many say .3 volt.

Now, as mentioned, is this the actual voltage at the ECM? Maybe the ECM just isn't "measuring" the voltage properly. It IS a value that is converted in an A/D converter, and maybe the ECM just isn't all that accurate.

I would check it at the the ECM pins with the car running, and if it is different than the scantool value, make sure the ECM connection is clean, and that all your grounds are good and clean, then dismiss it as normal, and keep in mind how far off "Normal" is.

JP

 

Doc was out studying cloud formations wondering what the tie in was between lightning and cars. At least he didn't take the kite out.... again

 

I'll do the voltage checks at the ECM connector tonight or tomorrow and report back. I've already cleaned a couple of grounds, will probably do that d*mn one on the back of the head tomorrow morning when I once again pull plugs to try and see if any particular cylinder is missing (more than any others?)....

I can't help but think of the KISS principle concerning my ignition miss. A miss is usually a bad (or dropped on the concrete) plug or plug wire. Looking at the plugs should tell me if its isolated to a cylinder or two. I've tried 3 different coils, have new cap and rotor, have replaced the plug wires first w/MSD then w/Taylors.
The engine miss at idle when warmed up and the 2000-2400RPM misses also seem to prevail across several different proms, both mine and 3rd party versions.

If they all look the same I'm starting to think about:
1. Distributor (it's the original 200K plus distributor most likely)
2. Module in the distributor
3. ECM (definitely the original, tho not factory prom(s)

Will keep you apprised.....

 

4. Injector?????

 

If the distributor is original check your reluctor. I chased a bad miss for a month and it turned out the reluctor was cracked causing an intermitent miss. One shiney new distrbutor with all associated hardware and my miss was gone. It is not always the problem but its really worth looking into and it costs nothing

 

Sounds like a good plan, razor.

Only thing is, I need some help doing that. What's the reluctor, where's it at, etc.

Appreciate the help....

 

It's the pole piece inside the distributor that sits above the pick-up coil.

 

The reluctor is below your rotor but above your pickup coil. Its on the distributor shaft. It is an 8 sided disc and about 2 1/2 inches in diameter. And if your distributor has 200K miles then it should be a rusty color. The easiest way to have a good look is to simply remove the distributor.

 

Trickster beat me while I was typing

 

SAWWY!!!!!!!!!!!!

 

Well, I think I found my miss yesterday.

After trying to set the IAC (and failing, I suspect I'm not getting the paper clip in there right), I started pulling plugs.

#2 was sparkling clean, didn't look like it was firing at all. Of course that was the 7th plug I pulled....

Checked the resistance on that UR5 and a couple of spares I had, and that plug was reading less than a fifth of what the known good plugs were reading resistance-wise. I had checked all the plugs before putting them in a week ago, and it was fine then. It just failed after installing it (or maybe got dropped?).

Anyway, threw it away, put in one of the spares, and I think I've gotten the problem licked. Weather cooled off a bit, so can't say for sure yet, but looking good so far...

Thanks to all for the help!!!!


- Vern

 

Well, I found out the source of my miss....

Pulling plugs a week ago showed #2 plug as clean as a whistle, almost brand new looking. Put another plug in there, no change. Was getting spark, compression checked out fine as well, so it had to be a fuel delivery problem, right?

Right.

Picked up a spare set of Z06 injectors to have on hand, and started removing the SuperRam plenum Sunday morning. Can't even see injectors for #5, 7, 2, and 4 with it on the car.

AS soon as I lifted the plenum up, I could see the problem. The #2 injector's electrical connector was not plugged into the injector! Apparently when we did the motor install in May we didn't get one of the connectors plugged on all the way and it worked itself loose.

Thanks for all the replies and input!!!


- Vern

 

 

I'm not thrilled with NGKs.
Too often they seem to lose the ash coloring deposits, and indicate a lean condition.

There is ALOT more science in plugs, then people want to realise. IMO, if one wants spark plug life then Autolites, for just consistant good performance A/Cs.

If you find any serious change from using a plug other then those, then you masking some other problem, IMO. While some plugs use a softer electrode material, that makes for an easier ion path for the spark to follow, and might cure a miss, they won't last as long, and you'll never see the problem.

Diagnostics is about curing problems, not covering them up.

I just base these conclusions on having looked at a couple sets in the last few decades, and Doc's been doing it longer then me, and he agrees.

 

Tell me about it, Morley - I know, I know!!!


GRUMPY ---

As for the Autolite or A/C's vs. the NGK's, what you're saying makes good sense. I've never used the NGK's before, but decided to try them since they were pretty short plugs (only 1/8th inch longer than the Accel shorty header plugs) and I have to use something shorter than the A/C's to fit due to the closeness of the headers on #6 and #8, in particular. Believe me, I'd rather use the A/C Delco's if I could, but I can't.

World Products recommends if using the A/C's to use the 43TS or 44TS plugs. I started out with the colder 43TS because of the 10.5 compression but burned thru 2 wires in less than a week (close enough to the headers could hardly even get the wire on the plug!). I'm also gapping them at .050" due to using an MSD Dig 6+ box and coil.

If I tried an Autolite, what plug and gap would you recommend? I've not been able to find one to compare lengths to see if I could use them.

Thanks to all for the inoput and assistance!!!!


- Vern