Fishbite diagnosis

[maverick2]

Great forum and website – probably the most useful of any forum of this nature I’ve seen! Started lurking after buying a ’94 Tahoe with a 6.5 in it to see what I could learn on its strengths and weaknesses. Only issue/question that has arisen so far is that of a very light “fishbite”. Have done various searches on “fishbites” and “PMD related problems” on this forum, and rather than reducing my level of uncertainty on order in which to attack things, I’ve probably increased my uncertainty. Sounds like “fishbite” can be hard to track down. With that said, I’ll lay out the symptoms and invite your recommendations.
(1) The fishbite is sporadic and very light – similar to an occasional “miss” in a gas engine. Very easy to mistake for a small bump in the road. Has never been any more severe than that – has never died, and has always started quickly and cleanly.
(2) The fishbite seems to be a lot more prevalent in hot weather – is NOT noticeable in the cool mornings, but is noticeable when things heat up in the afternoon.
(3) The fishbite seems to only occur under partial, steady load or partial, decelerating load. Have not noticed it when accelerating or pulling a hill/ load. (In fact, if it seems to “hiccup” a couple times in a row, providing more throttle seems to “cure” the hiccups.)
(4) Level of fuel in the tank is a non-factor.
Note, the rig currently has the stock PMD mounted on the pump.

Thoughts on likely cause, and where to start troubleshooting? Thanks in advance.

[More Power]

Yes, a fishbite can be tricky to pin down. Here is a list of possibilities.

1- You should have the interference trap on the harness pigtail installed on the fuel injection pump. This is a short extension harness that plugs into the connector on top of the pump. The trap portion of the harness is a metallic block of material that the wires pass through. Some feel the optical encoder signals are being degraded due to electrical interference. GM put one on there for a reason, but I doubt not having one is the cause of most fishbite symptoms.

2- The infamous PMD/FSD driver module could be intermittently dropping the signal. This driver module amplifies the computer signal to a level high enough to drive the mechanical fuel solenoid inside the fuel injection pump. I feel a flaky PMD could be a likely cause of fishbite.

3- The mechanical portion of the fuel solenoid inside the fuel injection pump is another possibility. The solenoid is driven 7200 times per minute when the engine is spinning just 1800 rpm. The close fitting components can be affected by poor fuel quality or a lack of fuel lubrication. If the fuel solenoid sticks, it'll seem as though the engine shuts off - for an instant. To test this theory, add a quart of new motor oil to the fuel at your next fillup. If lube is the answer to a sticking fuel solenoid, the oil should help reduce the symptoms of fishbite. If not, the problem is likely electrical/electronic.

Jim

[blackonblack]

I just solved my 6 month battle with a fishbite, stall, no start issue.
It started with the same "light" fishbite. It progressively got worse.

I initially replaced the stock PMD with a Standyne. It lasted 4 months and then it crapped out too. I now have replaced it with a DTech brand PMD.

If I were you I would by the remote mount kit and install a new PMD. This way you don't have to try to remove the old PMD.

Marty

[BIGTPM73]

This is the first time I have heard the word fishbiting, but it is a perfect description of what happens to me. I just had a whole new pump installed about 6 months ago, and I have the FSD installed on an aluminum heat dispenser on top of the engine. The truck has had no problems starting but this "fishbite" symptom has been a problem ever since I got the new pump installed. It only happen when I am at cruising speeds. If I give it gas to accelerate, the truck responds, but once I level off speed, the truck lightly jerks and shakes, best described as fishbiting. I have a spare FSD and I have switched them out, and the symptoms don't change. I will try the oil in fuel trick Jim mentioned, but could there be any other cause to this problem. I have also started to have the truck stall on me, but I think whats going on there is the wire harness that connects the remote FSD is bad. If it stalls, I open the hood, and unplug and replug in the FSD, and it starts up. I am right now going to order a new wire harness and see if that helps both problems. I am open to any other suggestions. Thanks Tom
98 k2500 6.5L TD

[Hubert]

If it stalls, I open the hood, and unplug and replug in the FSD, and it starts up. I am right now going to order a new wire harness and see if that helps both problems. I am open to any other suggestions. Thanks Tom
98 k2500 6.5L TD

Stalling and recovery with not much work is indicative of a FSD related problem either electronic or wiring. And with above statement seems you have to either eliminate FSD or wiring or both. Are you sure your spare FSD is good?

Just curious what is the "new pump" a rebuild or new Stanadyne? How many miles on timing chain injectors etc? I would suspect wiring problem from above just curious?

[BIGTPM73]

The truck has 215k mi on it. I am the 3rd owner. I don't think the timing chain has ever been changed, but I could never know for sure. The spare FSD is a new Dtech. I am going to reinstall that and see if it makes a difference. The one I have on there is a used Stanadyne. I got the pump, and both FSDs from Dieselcare.net, which I found advertised on Dieselpage. The pump was a rebuild. I had it installed by a mechanic who works on diesels at a GM dealer, and he took it into the dealership to have the pump timing and everything set. He can't figure out what the fishbite problem is, but I am going to try putting a quart of motor oil in the tank next time I fill it up. It really feels like it is struggling to get fuel and it chokes up, and the solenoid sticking sound like a possibility. As I said before, this problem happens with either FSD. I will let you know if the oil takes care of the problem. Tom

[BIGTPM73]

Well bad news. I switched the FSD to the Dtech and the truck runs terrible. It will start, but idles with a shake, blows out a ton of blueish black smoke, and won't accelerate when I press the accelerator. I switched it back to the Stanadyne, and it runs normal. I guess I have a bad DTech. Tom

[JFerg65]

Is the resitor in the new D-Tech?