RPM dip and surge

[malpana]

Hi, all. Checklist below. The RPMs often dip down to ~500 or so when any load is applied while idling or cruising with no acceleration (braking at idle or while coming to a stop, shifting into gear). It threatens to stall but doesn’t; the dip is followed by a momentary surge. When that’s not happening, it handles loads fine and idles at about 700-800. I’ve also had some harder warm/hot starts, but that’s not always a problem, and I haven’t gotten stranded yet; cold start is fine. The RPM dip was happening before I blew a head gasket and refreshed the engine about 500 mi ago.

New or reman parts during the refresh: rod & main bearings, heads, DS4, injectors, fuel sender, EP158 lift pump, glow plugs, PMD, OPS with extension hose, et al. I timed the pump and did a TDCO relearn: everything’s within spec. Anyone have a similar dip-surge problem?


Problem: Explain your problem in concise language (ie: Cranks but won’t fire). ___RPMs often dip to ~500 and then momentarily surge – doesn’t stall; warm/hot starts are often harder


Demographics: tell us about your truck and your ambient conditions: ___rural northwest PA, cold-ish winters, warm summers


- Year: ___1997
- Truck model and class (ie: c2500) ___Suburban K2500
- Automatic or Standard ___Auto
- Mileage: (indicate miles or km) ___347K mi (257K mi on factory replaced engine, 500 mi on refreshed engine)
- Replaced parts and mods (IP, PMD, chip/PCM, TM, injectors etc)___new relocated PMD, reman DS4, injectors, main & rod bearings, heads, starter, fuel sender, AC Delco EP158 lift pump, mechanical turbo waste gate controller, manual glow plug switch, et al.
- Model: ‘S’ or ‘F’ engine, NA or Turbo, EGR? ___F VIN, turbo, no EGR
- Ambient outside temp: (indicate *C or *F) ___avg 70’s lately
- What fuel are you using? (#1 Diesel, #2 Diesel, Bio, VO, WVO, etc) ___#2
- What fuel additives are you using? ___Opti-Lube
- Where are you located? ___NW PA


General maintenance: inspect and evaluate the condition of each. When were they last changed?
- Air filter ___500 mi
- Fuel filter ____500 mi
- CDR Valve? _____1 mi
- Coolant – last flushed/replaced at _______500 mi
- Oil – synthetic/regular, changed at ______400 mi
- Batteries – state of charge, (matched set) replaced at ____12.6V, 1000 mi
- Battery cables and grounds (removed, cleaned and tightened): ____500 mi
- Glow Plugs - how many miles on them? ___500 mi What type? ____Bosch Duraterm
- Injectors - last changed at ____500 mi




History: (Describe how the problem first appeared.)
- What were you doing when it happened? _____normal driving
- How was the truck running fine before this problem appeared? ____fine
- Has this problem ever happened before? _____Y
- Describe any related factors that might be part of the problem. _____See end of checklist
- Have you checked for DTC (Diagnostic Trouble Codes)? ___no DTCs




1] Starting:
1a] Does the engine start? ___Y
1b] Does the engine crank over? ___Y
1c] Are your batteries fully charged? ___Y
1d] Have you removed, cleaned, and tightened all battery connections? ___Y
1e] Does your Wait to Start light come on for a time? How long? __several seconds, then flashes several more
1f] Does your glow plug light come on? __NA For how long? ____NA
1g] Do you see smoke from the exhaust while cranking? What color? __N
1h] Does the Service Engine Light Glow during Start/bulb check? ___CEL always on since vac pump delete


2] Fuel Issues (if not running): VEHICLE RUNS, BUT I’LL ANSWER ANYWAY
2a] Can you hear your lift pump working? ___Y
2b] Do you get a steady flow of fuel from the water drain (T-handle) ? ___Y, >10 psi engine off, 9.5 psi engine idling
2c] Loosen the inlet line from the IP – is fuel coming out (Lift Pump on)? ___Y
2d] Loosen an injector line and crank the engine - does fuel come out? ___Y
2e] Do you hear a vacuum (hiss) when you remove the fuel cap? __Y


3] PMD / IP Issues
3a] Location of PMD (on pump, on intake, remote mounted) ___remote (bumper)
3b] Condition of Extension harness (if used) Vendor or Homemade? __vendoer
3c] Condition of grounds / ground wires to IP / PMD. Check Harness. __good, clean
3d] PMD Make: __Flight Systems
3e] PMD Age: ___2021, just installed


4] Stalling Issues
4a] Do you experience stalling only when hot? ___N
4b] Does it stall under particular circumstances? (ie hit bump, or on accel) ___N
4c] Does pouring cool water over the IP seem to help with restarting? ___NA
4d] Is stalling momentary (engine recovers)? ___NA
4e] Does SES light come on when engine hiccups? ____always on
4f] Does cruise control quit working when engine hiccups? ____don’t know
4g] Does re-starting seem to reset engine, restoring cruise and power? ___NA
4h] Changes to dash lights or instruments during stalling events? ___NA


5] Running Issues
5a] On cold start, does upper rad hose get hard before engine warms up? __N
5b] Any bubbles or oil film in coolant overflow tank? ___N
5c] Any white slime on oil surfaces (dipstick, CDR, oil Fill cap)? ___N
5d] Excessive white smoke from exhaust on cold start? ___N
5e] Excessive black smoke from exhaust under hard acceleration? ___N


6] Performance Issues
6a] Do you have gauges (boost, EGT) ? ___boost
6b] Maximum boost under load? ____~10 psi
6c] Have you added a performance PCM (reflash) or Chip? ___N
6d] Have you changed exhaust / downpipe / crossover? ___N
6e] Wastegate – mechanical or vacuum (OEM)? __mechanical


7] Temperature-related Issues:
7a] Single or dual thermostats? ___dual
7b] Have you replaced them lately? Manufacturer of replacements? __500 mi
7c] Have you pulled and cleaned Radiator and coolers lately? ___500 mi
7d] Have you replaced the fan clutch? Manufacturer of replacement? ___N
7e] (winter) Have you tested your antifreeze? Good to what temp? ___N
7f] (winter) Are you using a winter grille cover? ____N

[DmaxMaverick]

2e. How old is the fuel filler cap, and is it a "Diesel fuel" cap? It shouldn't noticeably "hiss" when loosened. A properly venting Diesel fuel cap vents very near atmospheric pressure, so it shouldn't allow significant pressure or vacuum. Too much vacuum allows air intrusion, while too much pressure causes return restriction. Either condition can cause throttle and power variations. Consider the fuel filler cap a consumable, and replace it periodically. If it's suspect, simply leave it loosened for a while (close it if you go through a car wash or use water pressure around it).

Your symptoms are typical for an early DS4 failure. If it did the same before replacing it, that is less likely. If the PMD/FSD has been relocated, where's the harness ground connected? If not screwed to the IP, it can cause issues, such as subtle or radical throttle and/or timing variations, due to the noise it can pick up from the chassis. Some IP's or FSD's are more sensitive to it, while some aren't. Try connecting the FSD (with resistor and heat sink, temporarily) to the original harness connector, without the harness extension. Failed or poorly made extension harnesses aren't uncommon. Some suppliers are better than others, but even the best can fail. If the IP is new-ish and failing, it wouldn't be the first, but eliminate everything else, first.

Note: Leaving a healthy wastegate solenoid electrically connected should clear the DTC causing the SES lamp. The PCM doesn't monitor vacuum, specifically, but will complain if a monitored component (EGR and/or WG solenoids) goes missing. What specific DTC's are present?

[malpana]

Thanks, Maverick. The filler cap isn't original, but it's fairly old. Should there be any audible vacuum at all? I'll loosen and try that. I'm hoping the reman IP I got from Pensacola isn't the problem - prior RPM trouble makes me think it isn't. PMD is grounded on the IP, and the connection is clean. I relocated the PMD about 12 years ago with the extension harness from Leroy. The only DTC is the complaint about the wastegate. Maybe I'll just put the solenoid back on so the CEL is meaningful again.

[malpana]

Oh, right, if it ought to vent near atmospheric, I shouldn't be able to hear anything.

[DmaxMaverick]

I'm sure you may hear something in the right-wrong conditions, but not regularly. It would probably take a completely failed cap to cause real problems, but it can, and has happened. The caps are cheap.

[JohnC]

The 12 year old PMD extension harness caught my attention... The symptoms are exactly what you'd expect from a failing PMD and the harness is the only piece that isn't new.

[malpana]

Thanks, John. I guess could check for consistent continuity while wiggling the connectors to see if there's a failing termination or wire.

[malpana]

Maverick, any recommendations for known good caps?

[DmaxMaverick]

Maverick, any recommendations for known good caps?

I don't know that it matters, so long as it fits and works. It's possible that all or some of el-cheapos are made by the same folks making the AC Delco caps, which I use (and pay a little more, just because). Prices vary $10-25, from your favorite parts store to Amazon, Walmart, eBay, etc, for the off-brands to AC Delco. If you intend to replace it regularly, the $10 model might be more appealing, and probably OK. The last one I replaced on my 2001 was about a year ago. The ACD cost about $18 from Amazon, and if I recall, only a dollar or two more than 10 years before. The locking version is a couple bucks more. Just verify fitment and that it's for Diesel. GM changed the design, gasser and Diesel, about mid 2000's, after about 20 years of the same fitment, so model year and trim should be verified.