6.5 wont go over 1500rpm when driving

[6.5erdydiesel]

Hey guys idk if people still talk about the old 6.5 but I have a 1998 6.5 turbo diesel it seems to idle decent but when you go to drive it it'll never get past 1400 to 1500 rpm and just falls flat on its face. Every once and awhile when it's in park or neutral it acts like it's starving for fuel and will drop to about 600 rpm then kick back up and idle fine. Any ideas on what could be going on? Replaced oil pressure switch back in fall but truck did sit for a little bit and also has stanadyne pmd and relocation kit. Hope to hear some good ideas

 

[Will L.]

Welcome.
Yes there is still a bunch of us here, and you just found the best source for this rig there is. Please fill out you signature line with truck details year make model, and any modifications done so everyone knows exactly what we are working with. Also any other details of recent work done besides the new pmd kit and OPS.

Use of punctuation and breaking up sentences makes it easier to read and concentrate on the issue- not that i liked English class in school-haha

Don’t buy parts until you mention where because there is a bunch of rip off places out there and we will stop you before you make same mistake we did if you mention.

About the OPS (Oil Pressure Switch)- was it an AC Delco and was it from an authorized dealer? The Delco is the best one we’ve learned and there is a ton of Delco knock off stuff online.
Even if the ops is bad or if the LP (lift pump) is bad it should still run up to max rpm when just sitting there.

remove fuel cap or caps if dual tanks and listen for it sucking in air. Start engine and try before pu
Any codes?
Have you checked the fuel filter?

Need to know fuel pressure getting to the ip. If you don’t have a gauge handy, long term it is a really great idea to install one permanent that you can see while driving. Low fuel pressure can cause problems but worst of all it can ruin the ip (injection pump)
You should have 8-14 pei, but no matter what never get to zero or a vacuum. you can check flow by having engine idle and open water in fuel drain valve. Catch the fuel that comes out noting how long it takes to drain a pint or quart. I prefer a glass container so you can let the fuel settle a few hours and see if any water separates. I am drawing blank right now on how many seconds it should take for a pint- ANYONE REMEMBER ? please chime in. If you crack open the valve and the engine dies- that means you have no lift pump pressure and it just sucked in air. You will have to solve supply problem first then bleed injectors of air, so I an not fond of this method because getting it running vs not running right is harder often. DO NOT USE STARTING FLUID!!! Besides blowing the engine, even if you get just the right amount it crystallizes glow plugs ruin

there is a 1/4” diameter 5” long return fuel oine coming out the front of the ip. Get clear line from any hardware store and 2 screw style hose clamps to fit it to replace the black hose. Takes about 5 minutes and costs maybe $5. Depending on the hose quality it might need replacement every 3-5 years but is SO worth having on there all the time. You are looking for air bubbles or any other contamination when it’s idling. Air bubbles means there is a “leak” in the fuel system but instead of leaking out fuel it is sucking in air. That can cause this exact problem.
Here is a video of the worst I ever saw yet the truck still ran. Full throttle would barely hit 1500 rpm. Older truck than yours. After the rotten rubber fuel lines were replaced, ran like a top. The ethanol/methanol in modern diesel fuel eats the old rubber lines from inside out. Must be replaced with sae30r9 or higher. YOURS DOES NOT need to be anywhere near this bad to cause problems, a couple of trickling bubbles the size of a bb is enough to make havoc. I need to post a video of a tiny leak in comparison but haven’t had opportunity. ((If ANYONE ELSE can, it would be helpful for guys- especially with a ds4 ip. Just loosen a clamp real wuick for the video... text it to me and I can post YouTube if you don’t know how.

A restricted air supply can kill rpm, but causes a bunch of exhaust smoke.

there is the possibly of a bad pmd or bad extension harness, cheap harnesses often fail. This guy is trustable.https://leroydiesel.com/products/leroys-flight-systems-pmd-relocation-kit/
Where did you move yours to? Many people make the mistake of leaving it under the hood where engine heat still gets to it. Back side of front bumper seems to be most popular spot on pickups. If hummer, then on top of the batteries is best.
either way I suggest a dual beat sink (or 2 regulars) with a spare known good mouned ready to go. Get a sealed dummy plug in the spare to keep contacts clean. Since only test is try new one, and especially if it dies in dangerous area, a quick move of the wire gets you back going in an instant. Having a spare pmd is like a spare tire- costs money but needed.

bad grounds on the engine can cause all kinds of issues for the pmd/ip. Remove, clean, reinstall. Especially the ones by the transmission dipstick. This should be every 3 year maintenance task for all you ds4 owners.

seeing as I just woke up, highly possible I missed some possibilities. Keep checking back today/ tomorrow and others will chime in. This site is super active.

there is tons you can learn here so take time to go through library and stickies. It can save a lot of money and heartache.

 

[JMJNet]

In reality, on a 98, the OPS should NOT be a factor in operation of LP.
However, a non ACDelco OPS may cause issues if it is the replacement.

Any DTC?
How is the fuel filter conditions?
What is the temperature in your area?

 

[6.5erdydiesel]

No lights come on the dash or anything haven't hooked a scanner up to see but when I first bought the truck the ops was bad and wasn't sending fuel to the injectors. Put a brand new filter in yesterday and still was having the same issue

 

[JMJNet]

If there is no fuel at injectors, then the PMD or the whole IP is bad.

OPS just controls the Lift Pump which carry the fuel from the tank to the Filter then to the IP, although on a 98, the relay suppose to do that with OPS is just a back up fail safe device.
Yes, the Lift Pump can cause fuel starvation.
That is why you need to read the long post from Will to determine what is going on.

Did you put ACDelco OPS? or you just bought one from local chain part store?
Local chained part store does not carry ACDelco OPS anymore, just their store brand which is junk, unfortunately.

Is the tachometer going erratic?

 

[tpattat]

I have a 1998 GMt400 6.5 four-wheel drive dually won't go over 1500 RPM blows white smoke I'm at a loss on it

 

[Will L.]

Does the smoke smell like diesel fuel or like antifreeze,

 

[tpattat]

Fuel it don't lose water or run hot .. I'm from mississippi but all way in Florida doing hurricane work .. would bad fuel cause this

 

[tpattat]

Also getting the code for injection pump timing. And the transmission was rebuilt but I'm getting a bunch of transmission codes too

 

[Will L.]

Remove and clean your ground wires.
Start with batteries and do engine. Especially passenger side rear.

Starting to sound like electronics is fighting for it.

Bad fuel would mess with it running right and smoking, yes.
Drain fuel filter hosing and catch what comes out to see.

Get a 1/4” x 6” clear line in place of the ip fuel return line - the one on the front that makes an arch. Just behind oil fill cap.
With it running watch for clean fuel with no air bubbles flowing is the desired outcome.
We tell everyone just leave it permanent. You have to replace it every 4-5 years but worth the $5 and 5 minutes.

 

[tpattat]

Ok thanks I'm going to go threw the grounds . Changed filter to no change