Surging?

[Logos5]

I don't know if this is the proper way to describe this problem. I have a 97 suburban, 250k miles. The truck starts ok. It idles ok in either park or when in drive. When I step on the accelorator the rpm's jump to 3000. I can not get the rpm's to increase at a steady rate, I have to let off on the accelorator. This happens in park or drive. I checked the fuel delivery system, new filter, pump works ok. I replaced the oil pressure switch ($30 and never been replaced). There are no codes. I used my scan tool to check the accelorator sensor voltage levels (ignition on, engine off) and all three voltage ranges check ok. I replaced the FSD around 40k miles ago. I checked the signals on the connector going to the FSD and are ok. Since there are no DTC codes I am not sure how to proceed to determine what is at fault. Any help would be appreciated.

[JohnC]

If it drives OK then ignore it. The software doesn't seenm to do well with off-idle speeds and no engine load.

If it does this while driving, you've probably got a pump problem. A scanner should show more about what's going on.

[Logos5]

JohnC,
It does this while driving as well, NOT just in idle. I am using the Autoenginuity scan tool. Are there specific inputs that I can monitor to diagnose the problem more? Thanks.

[JohnC]

First thing I'd do is give all the electrical connections and grounds a good looking over. Pay attention to the grounds on the rear of the pass. side head and the harness they come from. Those are the PCM to engine grounds. You may want to go through the stalling checklist on the DP. Also, verify the lift pump is providing steady pressure to the injection pump. If the fuel supply is fluctuating, the engine will surge and not set codes.

Next I'd look at the composite APP signal and see if it changes smoothly. If it does, look at the solenoid closure time and fuel rate. Usually, if the solenoid doesn't do what the PCM wants, the engine will shut down. If yours isn't, then it could be getting signals crossed up somewhere.

You might try a heavy dose of a good fuel additive, like FPFF or Stanadyne to see if things smooth out.

Where in upstate NY?

[robertb6963]

Same problem here.

Hope you make it out ok, let us know!!!

[Logos5]

JohnC,
Avon NY.

[Logos5]

JohnC,
I replaced the ground strap on the passenger side a couple of years ago. I checked it again and it looks electrically ok. I looked at where the wires go into the firewall that connect to the PCM. This "connector" is not a connector but a strain relief. Again everything that I can see looks ok electrically. I have tried the STanadyne injector fluid in the past, but will check again. I replaced the lift pump and checked the electrical connections. I did not check the pressure, but it appears ok (very good flow when purging air from at the fuel filter (also fluid is clear, no air getting in lines). I did look at the soleniod closure time and if memory serves me, it was around 1.58msec. If it is the injector pump then it will be a while before I can replace it. Any help would be appreciated, would like to save $$ and not have to buy a pump, but 250,000 miles is pretty good for 1 pump.

[Robyn]

Hello
Is your FSD/PMD mounted remote or to the IP ???

I have seen what you describe happen with a PMD thats going south.

You said the pmd was replace 40K ago.

Depending on where the little beast is mounted and such 40 K could have used it up.

Try another "known good" PMD If yours is not remote mounted it needs to be.

Many outfits sell extension harnesses for this purpose.

Before you get real deep into this thing I would try PMD/FSD

I have seen this before and the system will not set any codes even though the thing is totally whacked.

"Usually" if the IP has issues the computer will pick it up and set a code.

I have seen codes set for some of the errors that pop up and the rig runs fine.

Just some thoughts.

Let us know

Robyn

[Logos5]

robyn,
My PMD/FSD is mounted external to the pump on the driver side of the intake manifold. (I purchased the Deluxe FSD Cooler kit thru Kennedy Diesel). I checked all the signals at the connector and they are solid. I don't have a "spare" known good FSD. The one on the pump is shot. I called my local Stanadyne distributor and asked if he would check out the FSD. He said he needed the pump. I asked if he could hook it up to a pump he has and he started quoting me $80/hr. So I dropped it. Since I can remove the FSD if I had a way to test it I would. Thanks.

[gmctd]

Unfortunately, those symptoms are of Fuel Solenoid failure - scan-tool may not show that, but at 250kmi, you're way passed due for IP, injectors, timing chainset, prolly need to closely scrutinize the harmonic balancer and serp belt drive stuff - pulley, idler(s), idler pulley(s), and waterpump.