GM tech needs help, '88 Park Avenue VIN C

[Fred482]

Misfire on #3 cyl. 123,000 miles, regular maintenance, AC Delco parts. I have three 3.8's in the family, so I stock parts in the shop. I replaced the Magnavox ign module, Delco ign wires, plugs, coil pack & fuel filter mostly because of age & mileage. I did have one ignition module go out several months (about 4000 miles) ago. It quit in a parking lot, trailered the car to the shop, no spark, replaced the module, problem solved, until now.....

After "throwing parts at it" (against my better judgment, but it's my vehicle and I had parts), I tested fuel pump pressure and fuel regulator, both within specs, fuel delivery was good. I pulled the injectors, powered them up, spray pattern was erratic and one was leaking badly, sent them to a reputable outsource; they tried to service them, retested them and sent me a "reman" set. I cleaned and flushed the lines, installed a fresh filter and the reman injectors.

It started, ran on all six, smooth idle, within 3 minutes, #3 cylinder dropped out, dead miss. Pulled the plug, changed the plug and plug wire with a known good set on #3, same miss, same cylinder. When I pulled the plug, it was clean but wet with fuel. Tested the coil, it checks good, has good spark with the GM spark plug tester. Pulled the valve rocker cover, valve spring are good, rockers are oiling normally. Compression is good. Every time I start it in the shop, it runs smooth for a few seconds, then develops the misfire on #3.

During a road test, depending on throttle position and road condition, it will run smooth for a mile or so, then begin running rough and misfiring again. it's not consistent, scan tool shows normal readings on all sensors, but it's an '88 model, they don't display much data....

What am I missing? I'm at the "I can't see the forest for all the trees" point.....


Thanks, Fred

 

[HotZ28]

I noticed you tested the coil and have good spark with a spark tester. Were these test conducted after, or before the misfire begins?

 

[Fred482]

I noticed you tested the coil and have good spark with a spark tester. Were these test conducted after, or before the misfire begins?

Checked on both coil packs, after the engine was warmed and the miss was present. The first coil pack was an aftermarket NAPA Echlin unit, the one that is currently installed is a Delco replacement unit. Ohm readings are the same across the secondary windings on both units.

Because I used the GM service tool to check spark, I've ruled out the coils. The first gen coil pack is the three-coil module, not the individual coils that you can replace separately. I did swap plugs & wires several times. The miss stays on #3 cyl. The spark from the suspect coil terminal is strong and will stretch out to 3/4" from the wire when the wire is pulled up slowly.

The difficulty in this problem is that it seems to run smoothly for a few seconds, up to a minute or two, just after I start it cold, then the miss is present. It also seems to be intermittent when road testing. At times, it runs smooth, when under different loads (slight hills, up & down) and throttle settings. The scan tool readings from all the sensors are within specs. MAF readings are good, TPS, normal, ISC counts, normal, Block Learn & Integrator readings within range, O2 switches rapidly, "Rich/Lean" indicator light switches back & forth quickly, etc.

I've flushed the fuel system by running the pump, collecting a fuel sample, it's clean. Fuel purchases are from the same location as my other vehicles, I've replaced the fuel filter (twice) just to be sure of a clean fuel supply. The next step would be to do another injector drop test, to rule out a defective (stuck open) rebuilt injector, unlikely but I may do the test before going further in diagnostics. A simple fuel pressure drop test may reveal a problem....at this point I'm guessing again....🙂

 

[KBX500]

Hi Fred,
Sounds like you got a handful there. Taking into consideration everything you've said so far, this is what I'd be doing.

Run a compression check on cylinders 1-3-5 and compare them. I know a mechanical issue with cylinder 3 is highly unlikely, because it does run good for a bit as you said, but I'd check it anyhow. If it checks out OK you can cross that off the list.

Is there anything that taps into the #3 intake runner ? Vacuum port ?

Have you sprayed around the #3 intake runner checking for a leak ?

Is the ICM and coil on your '94 a single coil or three coil set up ? I'm guessing it's a three coil. You can see where I'm going with that. It'd sure be nice to have a known good 3.8 GM with a single coil to borrow a few parts from.

I'd also pull the coil and double check the connection to the ICM. Is it good on both the ICM and coil at the 3/6 position ?

How practical is it to pull the fuel rail and swap injector #3 with #1 or #5 ?

If I think of anything else, I'll post it.

Keith

 

[Kevandkaye]

I believe you said you switched coil packs with each other to see if the misfire moved to corresponding coil pack? Have you checked to make sure the coil plugs on the bottom of the coils are clean and tightly made on both the coils and ignition module, as KBX said?

 

[HotZ28]

Th Magnavox ignition is "the three-coil module, not the individual coils that you can replace separately." However this system can be easily converted to the DELCO ignition with the three separate coils. If the #3 plug is wet with fuel, the cylinder is not firing.

 

[Fred482]

Thanks for all the input, guys, I do appreciate it...!

This whole problem is typical for me. After purchasing the '94, I thought selling the one '88 with 208K was a good idea, it was tired, mine wasn't being driven much, I have a Fire District vehicle to use. I thought I'd sell the high mileage vehicle, move the wife into the lower mileage vehicle, eliminate one insurance payment, save the '94, use up the '88, etc. The wife travels quite a bit, 15K per year, she likes the '88, all good. When the misfire developed, I thought about 120K, threw a bunch of parts at it, hoping to eliminate future issues, instead of fixing the problem.....;-)

Correct, the '94 has the Delco system with individual coils, the '88 is the Magnavox system with the three coil pack, to replace one, you replace them all. Interesting note, I did convert my first '88 from the Magnavox to the Delco system a few years ago. I should have swapped it back prior to selling it a few weeks ago! That's where some of these "swap" parts came from. I remember it was quite a hunt to find the proper module bracket and a few of the other small parts.

I'm going to run another pressure drop test to see if one of the reman injectors could be stuck open, then pull the fuel rail (not difficult) and swap the injectors to see if the miss follows the injector or is just an ignition problem that's hard to locate. I may have to break out the Fluke 87 to measure injector pulses, that may tell me something too. If there are any intermittent signals at the module connector, it would show up on the meter.

I did spray the manifold for leaks, most of the vacuum ports are at the head of the plenum, the PCV port is closer to #2, on the other side, low down underneath the manifold. The PCV boot broke and caused a major vacuum leak a few years ago, it idled way up and ran rough, I replace it and the PCV valve, no problems since.

I did check the coil primary connections carefully for terminal contact when I installed the new coil pack. All connectors were tight & clean.

If all else fails, I have an old friend in the business who has a good scope. This may provide additional information. It should show ignition patterns and any deviations. I was just reluctant to drive it a few miles to his shop, not wanting to go too far with the misfire, it's probably a good alternative to "throwing more parts".

True statement, if the plug is wet, it's not firing. I do need to focus on the ignition, but because all indications and tests are saying there's good spark to the plug, I'm going to work on one problem at a time. Changing too many things at once is probably how I got lost in the first place. It is possible that one of the reman injectors is defective. The injector swap will help to verify a fuel delivery problem.

Thanks again, keep me posted if I'm missing anything. When this is resolved, I will post the findings for others.

 

[Fred482]

Thanks for all the input, guys, I do appreciate it...!
When this is resolved, I will post the findings for others.

Update....I swapped the injector to a different cylinder. The misfire stays on #3. I finally drove the car on the road for about ten miles. It ran smooth most of the time, whenever under throttle, any kind of load, it is smooth. Even stopped at idle, in gear, it's smooth.......for a short time! Sitting in gear, idling, it will start the misfire. I got out, pulled the wire from #3 at the coil pack, had my GM spark tester & wire already hooked to the ground stud, connected the wire to the coil, it's got a nice, steady blue spark, never missed a beat. Hooked the #3 wire back up, miss is still there. If I play with the throttle, rev it up, load it, etc., the miss will go away and it will run smooth again, until it returns to idle and sits for a few minutes. Sometimes, it takes minutes, other times, it starts missing after I bump the throttle with it in gear, idling.

Road testing, I can feel the misfire once in a while, but if I drive it normally on our rural roads, flat or hilly terrain, it only misfires a few times and if I push it harder, it smooths out again.

I performed an injector pressure test before swapping the injectors. It registered 42#, key on, engine off. Key off, it drops nearly 10 psi right away, bleeds down to 20# and holds steady. I know that's not right, but with the pressure gauge hooked up on the road test, the pressure runs between #35 & 40#, depending on vacuum/throttle position.

Side note: When I pulled the fuel rail, I left the fuel lines connected, Key on & off several times, looking for a leaking injector. The tips were all dry except the #3 injector. The tip was damp with fuel. After several times cycling the key, it did drip, one drop. I moved it to the #1 cylinder, put #1 injector in the #3 position, reinstalled the rail and went through the above mentioned road testing.

What have I found with all this? 😕

 

[journeyforce]

What was the compression test readings for #3? You have ruled out everything in the ign and fuel system. All parts that were attached to #3 were moved to other cylinders and worked fine. Could the sparkplug wire be grounding on something? Are those metal cups that go on the part of the sparkplug wire that attaches to the spark plug(to dissipate the heat) still there? If so did you try plugging in the#3 sparkplug wire without the metal cup/shield? Sometimes those things cause the wire not to seat right. What about the Crank position and Cam position sensors? A long shot but they could be causing these issues

So if all of that check out then that leaves the mechanical part of the engine as the cause of the problem. How is the intake gasket looking? A leak there could cause misfire issues. A small leak would be "fixed" when the engine warmed up and the intake expanded but a big enough leak would be there at all times

Good luck

 

[KBX500]

Well, regardless of the injector being damp, or pressures less than ideal, you did swap injectors and still have #3 misfiring, so you did find something. It's unlikely to be an injector or fuel related cause.

Just to eliminate something weird going on with the ICM/Coil package I'd beat feet for a junkyard and pull both, from a car that looks like it drove in and wasn't towed in. I've had too many issues with "new" ignition parts that later prove to be bad.

Have you tried pulling codes ? Even without the CEL/SES light on the computer can have stored codes. See what you have there, clear them out, drive it and check again after a misfire incident. You might be surprised and find something there. Hopefully there will be a code that will prove useful.

Have you done any wiggle tests ? Opened any looms up checking for wiring issues ?

Keith

 

[HotZ28]

Are you checking spark at the coil tower? If so, try checking it at the plug, using the ignition wire. ACDelco wires (made en Mexico) are not the same quality as they were in the past. I purchased a new set last year, where three out of six, failed within four months.

 

[Fred482]

Will check the items mentioned and report back.....thanks again for your patience and help! Fred

 

[Fred482]

Checked the compression on the front three cyls. All were at 150#, No coolant loss, no pressure buildup in the cooling system, The '88 apparently didn't have the metal sleeves over the front plugs, only on the three (#2,4 & 6) in the rear bank. I have exchanged several different plug wires and plugs in #3, just to make sure it's not a defective part. When I check for spark, using the GM tester, I clip the tester to an engine ground stud, put the tester on the plug end of the wire and leave the wire connected to the coil.

Road tested it with the scan tool hooked up, after checking for codes. No codes were set prior to or after the road test. I had run the engine, checked some connections, TPS signal, crank signal, checked the reluctor wheel(s) for tightness, checked the injector pulse width with the Fluke 88, etc.

After it sat for about 15 minutes, I started it, drove about three miles out and turned around. It ran on all six, smooth and normal, until about 1/2 mile into the return trip. I felt it missing, it got worse as I got close to the shop and as I drove up a fairly steep driveway (rough & rocky) slowly, it felt like it was dropping more than one cylinder. As I stopped to open the shop door, it died while idling. It started up easily, when I stopped in the shop, took it out of gear, put it back in gear, it died a second time. After restarting it, it smoothed out a bit, then it returned to missing on only #3 cylinder. I tried wiggling the wiring, working the connectors, etc., no change in the sound of the engine.

I'm off to a local yard to pull an ECM, just for fun, I've replaced just about everything else.

 

[HotZ28]

The ECM will not cause a misfire on only one (same) cylinder. You apparently have secondary ignition misfire, caused by ICM, CKPS, or coil pack.

 

[KBX500]

If you're going to go a salvage yard to get an ECM, be sure to get the ICM and coil pack also.

Hang in there Fred, there are just a few possibilities left.

Keith

 

[Fred482]

If you're going to go a salvage yard to get an ECM, be sure to get the ICM and coil pack also.

Hang in there Fred, there are just a few possibilities left.

Keith

Thanks Keith, I'm definitely still hangin' in.....just not about to hang myself yet...😉

I will be sure to check and, if it looks good, get the rest of it. I'll let you know what happens next, I feel like I'm in a soap opera....

 

[Fred482]

It's fixed....!! I replaced the ECM and installed a Delco system from a '94 Buick that I picked up this morning. It starts and runs smooth. I road tested and charged the A/C (it was 10 oz low on R12), drove it again for a total running time of over an hour, no misfire during that time. Before the fix, it would develop the miss in just a few miles/minutes of run time.

Thanks to all of you who were willing to help an old guy. I can't tell you how much it means to know so many of you, so much experience are willing to help. That's what makes me join these forums. Few rival the reception I've received here, I hope I'm able to assist in return. Thank you all!!

God Bless,
Fred

 

[KBX500]

Congratulations Fred ! I'm curious, was it the ECM or the ICM/Coil combo that fixed it ?

Keith

 

[Fred482]

Congratulations Fred ! I'm curious, was it the ECM or the ICM/Coil combo that fixed it ?

Keith

Hi Keith,

The ECM cured the engine misfire problem. I did an extensive road test, let it idle for long periods, turned it off, hot soaked it, restarted, idled, drove it again. No misfires.

Because I had converted the other '88, the one we sold recently with 208K, I was convinced I needed to up the reliability of this one, now that the wife is using it as her daily driver. I had picked up the Delco parts from a '94 Buick LeSabre yesterday, so I installed it after making sure the ECM was the original problem.

Mostly, I'm following the advice given in all the responses, including the repeat comment to upgrade to the Delco coil pack/module. It's a piece of cake if you know which parts you need from the donor, it only took a few minutes to install. The #1 & #3 plug wires need to be adjusted a bit for the difference in length, but that's minor. Ideally, you'd replace the secondary wires with the correct ones for the Delco coil pack, but I'm cheap and had just purchased the new Delco wire set for the Magnavox system.

Having experienced three prior Magnavox ICM failures in my long history with the 3.8 engine, I'm convinced it is, in fact, an antiquated unit and needs to be upgraded. I would recommend it to anyone who still has the early 3.8 vehicles. I believe, if you have a Magnavox ICM, it's not a matter of "if", it's a matter of "when" it'll fail. Probably an unfair statement, knowing how many are still running strong, just past experience, no offense meant...😉

 

[KBX500]

Hi Fred,
Thanks for clarifying that the fix was the ECM.

I can recall back in the mid to late 90s having ECMs throw us for a loop with regards to proper diagnosis of a variety of driveability problems. In the same time period Standard Ignition had a batch of bad, rebuilt ECMs that would complicate diagnosis even further. It got to the point that for awhile we just called the salvage yards for ECMs so we'd be sure of getting a good one.

You did the right thing changing out the ICM/coil pack. Those Magnavox ICMs were troublesome. Replacement ICMs had to be a money maker for the aftermarket for a few years.

Keith