Misfiring under heavy load

[jmgtp]

Noticed it on the way to work this morning... slowed down to about 50 on the highway in 5th gear due to traffic and when I gave it some gas to get back up to speed it was misfiring enough to feel it skipping and obviously lacking in power. Off the highway does it under heavy load as well (basically if you lug the engine at all). At higher rpm with a good bit of throttle it seems to not misfire at all and the idle seems fine as well. Mostly venting at this point - I'm getting a bit tired of the gremlins that come with an 18yr old car. Right now I'm assuming its an ignition issue and hoping its nothing more than a coil or the ICM since I have ample spares of both. I don't think it is the CPS or wiring since both of those items have been recently replaced.

I have had both the CPS and CPS wiring fail on me in the past. When that happens the source of the problem is obviuous since it loses the tach signal and you can visually see the tach spiking up/down with the intermittent signal. Tach was steady this morning so I'm assuming CPS/wiring are good. OBD I seems oblivious to the problem, no SES.

[bve32k]

Noticed it on the way to work this morning... slowed down to about 50 on the highway in 5th gear due to traffic and when I gave it some gas to get back up to speed it was misfiring enough to feel it skipping and obviously lacking in power. Off the highway does it under heavy load as well (basically if you lug the engine at all). At higher rpm with a good bit of throttle it seems to not misfire at all and the idle seems fine as well. Mostly venting at this point - I'm getting a bit tired of the gremlins that come with an 18yr old car. Right now I'm assuming its an ignition issue and hoping its nothing more than a coil or the ICM since I have ample spares of both. I don't think it is the CPS or wiring since both of those items have been recently replaced.

I have had both the CPS and CPS wiring fail on me in the past. When that happens the source of the problem is obviuous since it loses the tach signal and you can visually see the tach spiking up/down with the intermittent signal. Tach was steady this morning so I'm assuming CPS/wiring are good. OBD I seems oblivious to the problem, no SES.

To me this sounds like timing.

My car idled fine and reved up fine when it was in park or neutral. When it was i drive everything went wrong. When this was happening to my car on the freeway I would speed up and the car would keep on backfiring. 3 months before that I re-did timing, but the car never backfired for those 3 months. Almost a year later after replacing fuel pump, sending unit, coils, sensors, injectors, I finally re-did the timing and it solved all my problems.

[1988GTU]

^ Ricer



I would suggest timing like what we diagnosed with the white whale GTP posted above. Those bolts need to be tightened to there specific specs, no less!

[jmgtp]

I'm confindent its not the cam timing. There is a big difference between a backfire and a misfire. If this was a backfire then I certainly would be looking at cam timing.

It is definetly a spark issue. I replaced the ignition module/coils with another set I have and the problem is a bit less. It still has a slight occassional misfire under heavy load below 2k rpm. The ICM is not in the stock location and I think the issue may be a bad ground. When I swapped ICMs I probably unintentionally made a better ground connection and improved the situation a bit. It is mounted to the crossbrace where the stock coolant overflow used to be. The 12+ on the power connector for the ICM is using the stock wiring but the ground I had changed to ground against the aluminum ICM mounting bracket. Last night I grounded the mounting bracket to the chassis using some 8gauge - it now only measures a few ohms from the ground pin on the power connector to the front cylinder head so it is definetly a good connection. I haven't test driven it yet though so if it still does it then at least I will no it isn't a bad ground.

Nearly off topic side note: I took a spare coil pack from my Mazda and it fits nearly perfectly in the 3.4 spark plug bore. I began to salivate at the idea of coil on plug. The Mazda coils have a 2 pin connector... wouldnt it be neat to use the output of the ICM (2 pin) to fire a coil on plug setup! But, alas, the plenum over the rear bank shoots that idea down. Time for that custom short runner plenum maybe....?

[1988GTU]

At first bve's car would not backfire unless you rapidly feed it some fuel, but under normal conditions the car would have no balls, guts, or glory. Kinda like driving a four wheel moped with 2 fat chicks on the back. It was terrible!

Pull the inspection cover and check the timing, you'd be surprised on what you may find.

[bve32k]

I'm confindent its not the cam timing. There is a big difference between a backfire and a misfire. If this was a backfire then I certainly would be looking at cam timing.

It is definetly a spark issue. I replaced the ignition module/coils with another set I have and the problem is a bit less. It still has a slight occassional misfire under heavy load below 2k rpm. The ICM is not in the stock location and I think the issue may be a bad ground. When I swapped ICMs I probably unintentionally made a better ground connection and improved the situation a bit. It is mounted to the crossbrace where the stock coolant overflow used to be. The 12+ on the power connector for the ICM is using the stock wiring but the ground I had changed to ground against the aluminum ICM mounting bracket. Last night I grounded the mounting bracket to the chassis using some 8gauge - it now only measures a few ohms from the ground pin on the power connector to the front cylinder head so it is definetly a good connection. I haven't test driven it yet though so if it still does it then at least I will no it isn't a bad ground.

You should also sand off the paint and primer where the aluminum bracket and crossbar meet so it can get a good ground. I had my located there too and I had no problems at all.

But if you still have the problem I would look at timing. My car had lack of power too. I kept on telling my self it wasn't timing because I had just replaced all the timing stuff and re-timed the engine. I spent hundreds of $ and then said f it and retimed the engine. It fixed all my problems.

[1988GTU]

I kept on telling my self it wasn't timing because I had just replaced all the timing stuff and re-timed the engine. I spent hundreds of $ and then said f it and retimed the engine. It fixed all my problems.

Little voices need to be heard mofo!!

[jmgtp]

Timing is a valid point - and I do appreciate your responses - but I'm not at all convinced it is the cam timing. The engine runs perfectly, has all of its power, etc. But if you put a heavy load on it, like 4th gear, 1500rpm, and a lot of throttle it'll have a few misfire kicks as it picks up rpm - basically you need to be really lugging the engine. By a few I mean maybe 3 'kicks' and they arent too noticeable. I've had passenger in the car when it has done it and I mentioned 'its misfiring again' and no one else had notices. Above 2k rpm it runs flawlessly smooth no matter how your driving.

I did just replace the belt and I also torqued the cam gears to spec with a torque wrench. Of course now if it turns out to be the timing I'll have to lie about it! haha.

[1988GTU]

Check fuel pressure, could be falling off for a split second then kicking back up.
How old are the ign.wires and plugs?

[jmgtp]

Wires/Plugs are about two years old and maybe 6k miles. Plugs look ok still - I did pick up a new set and if all else fails I'll replace them.

I've already gone the fuel pressure route, it holds steady on the gauge. I need to drive it with the new ground wire in place but I don't think I'll get to it for another couple days.

[OldSkoolGP]

It sounds to me like a faulty coil or a plug wire arcing to ground. I had the same problem when I rebuilt my car the first time. It was a cracked coil. You should check your coils over carefully. If you have them mounted in the factory position below the front manifold, heat will cause them to expand and contract very slightly, and evantually they will just crack. And bve is right about the coil bracket. If you painted your block, you need to scrape off the paint where it bolts to the block to ensure good grounds. Also, thoroughly check all of your wires and make sure the insulation is all there with no cracks or melted areas.

[jmgtp]

It is grounded now from the aluminum bracket to the bolt that holds the crossbrace. I measure only a few ohms of resistance from the aluminum bracket to a cylinder head so the ground is good - I have not yet test driven it since making this change.

I'm hoping it was the ground but the fact that it got a lot better once I swapped ICMs/Coils may indicate an ICM/Coil issue. I think the reason I have so many spare ICMs/Coils is because they failed on me and I kept picking up spares. Since I foolishly don't throw out bad parts I probably put another bad ICM/Coil back on. The test drive will tell.

I'm probably going to put the new plugs in this weekend and at least visually inspect the wires, coils and ICM.

[Juglenaut]

first thing try not to rely on resistance from the engine to the ICM bracket as resistances can be misleading no matter what you do because the reading you are getting its the path of least resistance and other locations may actually have worse to where the spark may not travel its the specified path.

The best way to find loops is by voltage drops both AC and DC, locate different areas on your engine front to back, battery to ICM, to engine, to body in both AC and DC two point of commonality should be very close to zero. Check for noise coming up from the CPS. My valid guess is you are dealing with some noise from somewhere.

A simple way to check for electromagnetic interferance is to set a portable radio to a pink noise frequency on AM basically no sound and hold in in areas around the engine see if you get any pinging on the radio. The ICM should be grounded to the block.

[jmgtp]

I found 2 spark plug wires have ~1/2" cuts in the silicone surround - so my new theory is faulty plug wires. Luckily the cuts are right near the coil end boots and I should be able to trim the wire back and put new terminals on with plenty of wire length to spare.

I want to eventually make a new set of wires using the 96 boot ends. My rear bank has the 91-95 style boots with the 'T' shaped handle cut off to clear the 96 manifold.