Updated: These spark plugs are not the misfire you've been looking for.  It was a dying fuel pump.

"ITA97, now with more Jag @ opposite-lock.com" (ita97) 10/25/2016 at 17:56 • Filed to: None

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It turns out the fuel pump was giving it up. Fuel pressure was initially within spec at idle and under the “simulated” WOT test procedure in the FSM that consisted of plugging the vacuum source for the fuel regulatory and looking for a rise in in fuel pressure to the 43-47 psi range.

After the fuel pressure initially checked out, I poked around some more looking at ignition connections and troubleshooting flow charts. I found nothing, so I decided to recheck the fuel pressure. This time I secured the test gauge under a windshield wiper so that I could see it while driving. Under load fuel pressure would drop, and under actual WOT is was dropping under 20 psi. The engine was leaning out and inducing the misfire and hesitation, but it was not staying in that condition long enough to set a code. By time I finished the second test, the fuel pump was down to 10 psi at idle, which is not enough to actually run the car.

To confirm that there were no electrical problems, I powered the pump directly off the battery via power probe and confirmed it would still produce no more than 10 psi. While I don’t have the adapters to deadhead the fuel pump, I did briefly power it up with the supply line disconnected tho confirm that it only put out a very gentle stream of fuel and that the fuel filter I replace 8k ago isn’t clogged up. At least a whole new sending unit is less than half the cost of the opti-spark I thought I was going to have to buy, and it had the good sense to die completely in my garage while I was troubleshooting it instead of on the interstate tomorrow morning.

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Over the last week, the corvette has developed a misfire, especially between 3-4k RPMs. The problem is often non-existent when cold, usually present when hot, and gets much worse with prolonged idling when hot or on a hot restart after being parked for relatively shorter periods of time. This has all the markings of the dreaded opti-spark failure (and the car is at 78K on the original 1993 opti-spark), but the corvette is not being kind enough to trigger any ECM codes. The ignition troubleshooting charts lead me down a path that says everything is fine (and I don’t have an OBD 1 reader that would let me see the high and low resolution pulses in real time), however the corvette forums seem to have plenty of folks who have had opti-spark failures that never triggered any codes.

As a starting point, and because the car was still allegedly on the original spark plugs at 78k and 23 years old, I took the opportunity yesterday to put in some new plugs. 6 of the sparks plugs are no big deal after you move the ASR module out of the way. The other 2 are a lot less fun. Plug #6 is not fun. #8 made me think bad thoughts towards GM as I turned my good, snap-on ratchet one mother-loving click at a time. The possible range of motion on my lesser Craftsman Professional ratchet was a no go, as it was less than a click. I’ve heard the F-body cars of the 90's were even worse. I was hoping (but not expecting) to see some plugs with with big, eroded gaps or a broken insulator that would explain a misfire. Instead, I got 8 plugs that looked pretty darn good. Curiously, the records never show the plugs to have been replaced and they were the correct OEM parts, but the gaps on every plug was substantially smaller than spec. The car calls for a .05 gap, and all 8 in it were all gapped around .03-.035. Between the incorrect gap, and the better than expected condition for 78k, perhaps I am missing a service record for someone changing them out sometime in the 22 years before I owned the car. What I didn’t find was any thing that would explain the misfire.

I replaced the plugs with new, OEM Delco double platinums. I even gapped them to the correct spec. When not misfiring, the car purrs nicely. I did spot the beginnings of a bit of corrosion on the terminals in a couple of the wires, but nothing that would explain the problem. Nonetheless, I did order a new set of wires.

On the fuel-side, the fuel filter is pretty new. I changed it earlier in the year, or about 8k ago. The injectors all test in the range on resistance, they are all clicking nicely when running, and I see no signs of leakage into the vacuum system at the fuel pressure regulator. However, I don’t actually own a fuel pressure tester. While this doesn’t feel like a fuel-side problem, I’ll probably take this as the opportunity to buy a fuel pressure test kit before coughing up the dough and time to change the opti-spark.

crowmolly > ITA97, now with more Jag @ opposite-lock.com 10/25/2016 at 18:01

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ICM/Coil on the head? Possible heat soak?

Just throwing it out there since the “mod” is significantly cheaper than a new opti.

LongbowMkII > crowmolly 10/25/2016 at 18:04

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That just replaces the rotor/cap of the opti. The sensor is the important bit.

crowmolly > LongbowMkII 10/25/2016 at 18:08

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Uh, what? I’m talking about shimming the ICM/Coil if it’s on the head like the f-bodies.

BrianGriffin thinks “reliable” is just a state of mind > ITA97, now with more Jag @ opposite-lock.com 10/25/2016 at 18:08

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Would there be any reason to suspect a vacuum leak of some kind?

LongbowMkII > ITA97, now with more Jag @ opposite-lock.com 10/25/2016 at 18:08

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Frankly, I found the optispark was easier to change than the plugs. (B-body, YMMV) it’s the paying for the opti that’s the painful part. Fortunately I was able to clean mine out (MSD replacement) and replace the oil seal.

Any appreciable leakage from the water pump drain hole?

ITA97, now with more Jag @ opposite-lock.com > LongbowMkII 10/25/2016 at 18:20

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Not at the moment, but the water pump did leak out onto it last year. As soon as I figured out that coolant was weeping out the hole, I stopped driving for a week until I replaced the water pump the following weekend.

gogmorgo - rowing gears in a Grand Cherokee > ITA97, now with more Jag @ opposite-lock.com 10/25/2016 at 18:21

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It sounds a lot like what the MJ started doing before the fuel pump died. Fuel pressure gauge is a good investment, went fairly quickly from small coughs under heavy load to significant power loss (and backfiring, presumably out the intake) if I pushed the throttle at any rpm above 2000. After cooling down, even for only long enough for a fuel or fast food stop (or it just occurred to me the fuel pump might have been cooled by the new fuel, derp), it would be good for a while, and as it got hotter the problem got worse. When I say it progressed quickly, I should categorize that I was on an extended highway road trip, so even though it went from bad to worse over the course of two days, it was still something like 1000 miles.

ITA97, now with more Jag @ opposite-lock.com > BrianGriffin thinks “reliable” is just a state of mind 10/25/2016 at 18:22

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I didn’t put a vacuum gauge on it, but I did visually check out all the lines and connections. I didn’t see anything suspect, and I would expect a vacuum leak to cause it to have less intermittent issues.

LongbowMkII > crowmolly 10/25/2016 at 18:24

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Ah, thought you were referring to a mod that uses the N* coilpack

crowmolly > LongbowMkII 10/25/2016 at 18:26

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Nope. Was talking this one:

http://shbox.com/1/4th_gen_tech2.html#ICM_cooling

ITA97, now with more Jag @ opposite-lock.com > crowmolly 10/25/2016 at 18:29

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For less than $150, I am tempted to order a new ICM and coil before buying a new opti.

crowmolly > ITA97, now with more Jag @ opposite-lock.com 10/25/2016 at 18:31

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What are you using to scan? Datamaster?

Eric @ opposite-lock.com > LongbowMkII 10/25/2016 at 18:31

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Yeah, the problem is changing the wires and plugs. The optispark is accessible, it’s more the cost, annoyance, and water pump tending to make it an issue than the efforts changing it. That water pump mounting was the big problem with the optispark.

Based on his problems, it doesn’t sound like an optispark problem, though. Usually you notice the misfire when stopped or throughout the RPM range because there’s some water sloshing in the bottom of the optispark.

This sounds like something else breaking down to me.

ITA97, now with more Jag @ opposite-lock.com > gogmorgo - rowing gears in a Grand Cherokee 10/25/2016 at 18:31

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Indeed. I will drop $40 for a fuel pressure gauge before I drop $600 for a new opti and water pump.

LongbowMkII > crowmolly 10/25/2016 at 18:37

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Neat.

ITA97, now with more Jag @ opposite-lock.com > crowmolly 10/25/2016 at 18:42

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Reading codes via the CCM display. It says all is well, and it behaved as it should in test mode. I don’t have an OBD 1 reader.

crowmolly > ITA97, now with more Jag @ opposite-lock.com 10/25/2016 at 18:45

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Gotcha. Logging data would make this a lot easier, but you probably already know that.

ITA97, now with more Jag @ opposite-lock.com > ITA97, now with more Jag @ opposite-lock.com 10/25/2016 at 19:14

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Thinking about this some more, the misfire is mostly happening under harder acceleration. A gentle rev through the RPM range is usually fine. Giving it the beans will induce the misfire, with a whole lot of hesitation between 3-4k. It is fine in steady state cruising. If fuel supply is insufficient and it is leaning out due to a fuel pump on the way out, the filter clogged (unlikely given that it isn’t very old) or the regulator having a problem, it would make a whole lot of sense. I’ll see if I can pick up a fuel pressure tester on the way home tonight.

It does have a fuel pressure sensor, and it should be able to set a code for low pressure or a lean condition, but I also read today that the ECM has to see an error for a certain amount of time to set a code. It may not be leaning out long enough to set the code.

ITA97, now with more Jag @ opposite-lock.com > gogmorgo - rowing gears in a Grand Cherokee 10/26/2016 at 02:15

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It did turn out to be the fuel pump. It was within spec at idle and the FSM’s simulated WOT test, but was dropping pressure under actual load and leaning out. I first really noticed the issue last week. Between a trip to ABQ this weekend and a week of driving to work, it was about 750 miles. It actually gave out as I was testing it the second time tonight.

Eric @ opposite-lock.com > ITA97, now with more Jag @ opposite-lock.com 10/26/2016 at 05:21

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Good man. You often don’t notice it until it is too late and has been dripping hot coolant on it while driving for quite a while, which destroys the seal, then the optispark.

bhtooefr > ITA97, now with more Jag @ opposite-lock.com 10/26/2016 at 05:35

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Yeah, I made the same mistake on my Miata last year... Twice.

Replaced the coils and wires, because they always go bad on NB1 Miatas, right? Seemed to clear it up... Until the car started running like shit again, until it died a quarter mile from home. Because I had gotten no-name coils from Advance, I decided to get real Mazda coils... And that didn’t fix it.

But, wait, it’s running on starting fluid? And the fuel pump is getting power? Damnit, I bought the second set of coils for nothing, didn’t I... (At least I’ve got a backup set now?)

gogmorgo - rowing gears in a Grand Cherokee > ITA97, now with more Jag @ opposite-lock.com 10/26/2016 at 10:00

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Yeah, the Jeep kinda did that too. The fuel pressure test is always at idle, pulling the vacuum line off the pressure regulator to simulate pressure at WOT, but that was testing fine. Then out of curiosity I revved it up with the regulator unplugged and the pressure dropped off the gauge, and the engine stumbled pretty bad. That’s when I knew I needed a new pump.

ITA97, now with more Jag @ opposite-lock.com > crowmolly 10/26/2016 at 10:34

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Indeed. Data logging would’ve saved me some time. I would’ve seen that it was leaning out under load, and I would’ve not looked at the ignition system at all.

crowmolly > ITA97, now with more Jag @ opposite-lock.com 10/26/2016 at 10:38

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Just my opinion, but it’s probably worth digging around to try and find a copy of TTS datamaster. You can pair it with a $50 cable and it will greatly help diagnostics.

Datamaster was offered free for a while (for 20 scans) and then you’d have to purchase or uninstall/reinstall.

Or give Scan9495 a shot. Might be good, I haven’t used it.

ITA97, now with more Jag @ opposite-lock.com > crowmolly 10/26/2016 at 10:54

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Thanks. I’ll look into that.