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6.5 turbo fish-biting at highway speeds

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47Rishton

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Greetings, I have a 1994 2500 pickup, 4x4 with the 6.5 turbo and auto. It wants to over rev on a cold start to 2000 rpm's, then will idle down to 600 when it warms up. I read that it could be the temp sensor in a discussion. Will replace that. My bigger issue is that it pulls great and doesn't smoke on acceleration. Will pull like a demon to whatever speed you want, but when you let off the accelerator and just want to cruise, the engine rolls, or as some call it, fish-biting. It continuously surges down the road. If I decelerate it quits and when I accelerate it stop surging but if I try to maintain a steady speed, then it starts again. I replaced accelerator pedal, first year of fly-by-wire. Air intake temp sensor in plenum, fuel filter, although it only had 3000 miles on one I replaced. Also new PDM, which is mounted on right fender, in front of oval hole and replaced the sensor on the firewall, I think it's the MAP sensor. If anyone has had experience with, and fixed, either of these issues, I would be grateful for you assistance. The things I replaced were small dollar items and I hope to eliminate potential side issues. Next stop is some shop but I cringe at thinking about them just swapping expensive parts until it'd fixed. Thanks in advance.
 
First question, how many miles on the truck and how many miles on the Fuel Injection Pump (IP). The IP has an expected life of 150K miles, but with the ULSD being used for the last decade-plus, unless you're adding a lubricant at every fill up, the internal wear on the IP is greatly accelerated. The symptoms you describe are the same as the IP wearing/worn out.
 
@47Rishton The first test for an IP wearing is the "Steady Pedal Test" with the truck fully warmed up, idling and sitting in Park in your driveway, slowly depress the accelerator pedal and bring it up to 2000 RPM and hold the pedal steady at 2000 RPM. If with the pedal steady, the RPMs fluctuate that's an indicator that your IP is internally worn and on the way to runaway motor status, that's only if the fishbiting hasn't gotten to the point where it's 50-100 RPM and causing the TCC (Torque Converter Clutch) to unlock-relock-unlock-relock as the RPMs change -- especially noticeable on long downhill grades with the cruise control on -- you can feel the vehicle lurch as the TCC cycles through unlocking-relocking. That WILL burn out your transmission if allowed to continue for several thousand miles/months.
 
Before condemning the IP you need to check fuel pressure to the IP. Being as yours is a 94 the LP (lift pump) is controlled by the OPS (oil pressure sending unit). The OPS has 2 sets of contacts in it, one for the oil pressure gauge and one for the LP. Very common on 94-95 to burn up the contacts for the LP and the oil pressure gauge in the dash to read fine. You need to check fuel pressure at the IP while experiencing the fish bite. Fixing this has solved alot of fish bite issues. Several vendors sell a relay kit to take the load off the OPS or make your own.
 
I agree with our friend in Alaska- get a fuel pressure gauge on it. Low liftpump supply will ruin a good new ip, as well as causing issue similar to your description.
My suggestion is put a “T” in just before the injector pump with steel fittings. A damaged line before that can cause the issue and you are reading pressure back near the filter that looks good. Seen several people replace good pumps over this.

The other thing that is $5 and 5 minutes is replacement of the 1/4” diameter 5” long fuel hose coming out of the injection pump with clear tubing. You can get it at any hardware store and get 2 screw style hose clamps at same time for it.
You can see bubbles indicating air intrusion, water or other contamination. This is actually the first step GM says to do in diagnostics.
 
Thank you, folks. I will perform all the checks that you have suggested. My truck has just over 200,000 on the odo and I have only seen few parts replaced before I got it. I replaced the vacuum pump right after I got it as it had none. Also silicon vacuum lines. Don't know what was changed or when. It was used by a young man with a tree service. I found receipts for $1500 he spent on wheels and tires. Should have invested it in an injector pump and associated components. Many thanks again.
 
Let us know how it goes and take the time to read the stickes. Tons of money and time saving stuff in there
 
I printed the replys so I can have them in hand when I start my diagnostics. Thanks.
 
47Rishton said:
I found receipts for $1500 he spent on wheels and tires. Should have invested it in
Click to expand...

Krap tires blowing out or loosing traction will ruin your day. Nothing ever wrong with good newer tires.

X2 as it wasn't clear. Do NOT THROW PARTS AT THIS!!! It will create more problems and waste money. Diagnose it, ask on here, but never, ever, invest in parts just because you are guessing and HOPE. (Unless it's a new and spare PMD and USA made cable.) Then and only then a DB2 conversion is a better choice.

The DS4 will fishbite just once now and then just to piss you off, period. It's actually a test to see if all the sensors are working. Your case is excessive and a problem.

X2 on checking for lift pump working and air in the IP return line as mentioned.

You should pull codes just in case there is one that doesn't trigger the light. Some esp. transmission and all non-emissions related things don't trigger the light.

How many miles on the injectors if you know? These are a 100K service life item for the good ones. Good luck getting 30K out of China ones.
 
Sorry, I don't know anything about the injectors. Glow plugs seem to work good. As soon as light goes out it starts right up. It ran like a watch, then one day started the high idle when cold and the fish biting at the same time. I had to drive it to work one week a while back. Never seemed to get any worse or better. Was 70 miles to my hotel and 6 days of a 40 mile round trip to plant from hotel. Drove it home while doing the same thing but will accelerate like a scalded ape then start fishbiting when you try to maintain a constant speed like 60 to 70 MPH.
 
just so you know they aren’t confused as to why would it be there in the first place:

It was for better reception on AM radio being transmitted analog. It stopped the electrical noise from the injector pump to the radio.
So if you plan on listening to am radio in a third world country, maybe keep it in the glovebox for that day! Haha.

I have found in almost every case like you have there was air/water in the fuel or bad lift pump pressure/volume.
But don’t just throw a new lift pump in- new defective is becoming common place and only way to know is a gauge.
 
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