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Discussion Starter #1
1. Describe the problem you are having in detail:
Check engine light on various codes - starts and runs but is in limp mode. suspect bad computer or optical sensor. Starts and idles everytime. NO air on return line to IP.

2. Year of truck/engine.
1995 Chevy Tahoe 2 dr.

3. Odometer reading (indicate miles or kilometers).
96K

4. Indicate the model number on the Injection pump (starts DS4 or DB2......).
Believe to a stock unit - believe to be a ds4

5. Indicate if you know if it’s a 1500, 2500, 2500HD, 3500, 3500HD.
1500

6. Do you have an EGR on the engine? (An F or an S engine code 8th VIN digit)
YES

7. Air Filter condition (visual check).
Upgraded to high flow stock unit

8. Fuel filter condition (freshly changed, mileage since changed).
Freshly changed

9. Location of PMD/FSD? (ex. on pump/remote over intake, behind bumper)
Brand new PMD behind bumper with entsion harness - kit from ebay
.
9a. If remote mounted, describe wiring harness (homemade or purchased from which vendor).
Ebay purchase

9b. Indicate the location and condition of the FSD/IP grounding wire.
ground wire for IP on top cleaned and grounded.

10. Outside Temperature (C or F). _____ °
60 degrees F

11. Service Engine Light while running?- on/off/intermittent
On all the time - at first was on only after hard accelration now all the time.

11a. Service Engine light does glow during start/cranking/bulb check: Yes/No
YES

12.Have you scanned for engine codes? Yes

12a. List exact results on engine codes.
17, 18, 33, 35, 36, 54, 92, 96

13. Condition of Battery terminals (removed, cleaned and tightened).
Clean and tight

14. Known condition and age of Batteries. New

14a. Are batteries a matched set of same age? New

15. Condition of Major Grounds (removed, cleaned and tightened)
Cleaned and tight

15a Have the batteries been individually load tested? yes new

16. Does engine crank, or "turn over"? Yes

16a. Does engine start and run? Yes

17. If engine does not start- Crack injector line: do you have fuel? Yes/No

18. Does your Wait To Start light come on? YES

18a.Number of seconds WTS light is lit. 5-10 sec

19. Engine Cranking speed (if you have an accurate tachometer). good n o issues

20. Are you experiencing Stalling? NO

20a. Describe the event (upon startup only, down the road, hit a bump, etc...)
Truck fires everytime no issue - idles loudly. No power and is in limp mode.

20b. Do you notice loss of dash or instruments? NO

21. Check turbo inlet and air filter for obstructions. YES

22. In a no-hot start, pour a bottle of room-temp water on the injection pump. Does it start now? NA

23. Lift pump test - Describe results. Not performed

24. Upon cold start, does the radiator hose get hard quickly? No
25. Upon cold start, do you have excessive white smoke? No
26. Do you have excessive cranking time before the engine starts? No
27. Have you used the block heater? Does it affect engine starting? (only try for starting problems). NA

28. Are all glow plugs in proper working order? As far as I know

29. During hard acceleration, do you have excessive black smoke? No
30. Do you have any unusual exhaust smoke issues? NO
31. Turbo check out - Pass/Fail - not completed
32. Indicate fuel that you are using: Bio-Diesel, #2 Diesel, SVO/WVO, other
Standard #2 i believe - i'm from NY.

32a. If running Veggie Oil fuel setup, indicate details of your conversion (homemade or packaged system). NA

33. Are you using any fuel additives? If so, please list. NO

34. Upon unscrewing fuel cap, do you have a large vacuum formed in the tank? not sure

35. Do you have any service history available that might pertain to the problem you are having? NO prior service records

36. Please indicate any modifications to the vehicle that might help us diagnose better.
straight piped is all.
 

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3 of those codes lean towards a possible PMD and/or extension harness issue(s)
Start by ruling out a bad PMD extension harness.
Clear those codes then..
Disconnect the PMD from the ext. harness and connect the PMD to the OEM harness on the IP and see if things change.
If you see an improvement then you need another harness

Did you go through all of your grounds using the link for reference in the FAQ?
If not, go through them all. All it takes is one bad ground to cause a lot of issues in a 6.5L

The Ebay harness( depending on seller) can be China junk.
Try another "Known working" PMD w/resistor on the OEM, IP harness as well.
 

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17 pcm or optical sensor(high res circuit fault)
18 pcm or optical sensor(pump cam ref fault)
Could also be the 5v ref signal or some open or shorted circuits.

33 vacuum leak or failing vacuum pump (baro hi / low vacuum)
Could also be related to the solenoids on the driver bank. Or their harness connectors.

35 pmd (pulse width injection / pwi time short)
36 pmd (pwi time long)
Or as dually suggested it could be the extension cable.
It could also be the 5v reference signal or some bad grounds. You said the pmd is brand new? Bumper mount?

54 bad ops or lp (fuel circuit loop fault)
Could also be a bad 5v ref. Or pcm.

92&96 bad injectors @ cylinder 2 and 6 (cylinder balance faults / misses)
Both are on passenger side. 2 is in front. Even the rest of the way towards back. 6 is 3rd one from the front. 8 is in the rear. Odds are on driver side; 1 3 5 7. Check for air in the fuel system. Bleed the fuel filter by opening the bleed valve on top. Use a clear hose into a can. So you can see air bubbles. Air in fuel is a major no no. Check the return lines AT the injectors listed as well. May be leaking. If you dont have air intrusion and the return lines on injectors are fine; replace injectors #2 and 6.

There are some grounds @ rear pass side of engine. Clean those and make sure they're good. The ground strap is on the firewall nearby. Make sure it's good. As well as the ground to the frame. All on pass side.

Make sure the lp is flowing. When bleeding the air from the filter (with the truck on) if it stalls you need to replace the lp and / or the ops... or the ops harness connector is bad.

Being a 95 model year (i have a 94 model year 2dr Blazer... identical basically) you need to perform the "94 & 95 lp ops mod" use the search box and that term or maybe dually would kindly post a link.

I went thru this maybe a week or two ago... this might help walk you thru some of it...
https://www.dieselplace.com/forum/63-gm-diesel-engines/21-6-5l-diesel-engine/978326-one-has-me-stumped-guess-its-time-year.html

You're gonna need this too... (and a multimeter)
https://www.dieselplace.com/forum/attachments/63-gm-diesel-engines/21-6-5l-diesel-engine/342202d1429922680-6-5l-faq-s-master-list-information-6-5-answers-1995-obdi-dtc-codes-trouble-shooting.pdf

You should ditch the egr system. Its simple. Remove the vac lines to the egr, reroute accordingly. Leave the boost solenoid in the vac circuit. That's the 1st one from the front. Leave the baro sensor in the circuit (that's on the firewall) remove the intake. Replace the intake gaskets with the fvin / non egr gaskets. They have a block off for the egr. :thumb: you'll loose some mpgs though. But not much.

Given all the issues; just in case... I'd also inspect the harmonic balancer and crank pulley for issues. The low end on these engines are inherently weak. Last thing you want is a broke @$$ engine. The rubber insert on the balancer should be flush, no protrusions, cracks or missing chunks. The assembly should be nice and firm. Solid. Same for the pulley, no cracks or missing chunks in the rubber. Inspect the metal arms for wear where the mounting bolts may be contacting the metal. Itll be obvious.

I've got a fairly busy day today so unfortunately I wont be on as much today. I'll check back in every so often.

:thumb: hope this helps...
 

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As said check grounds before replacing parts.. Many of the codes can be related to a bad ground. Main ground for the PCM and sensors related to your codes is on the rear passenger side intake manifold bolt. Should have 3 wires going to it. One thick 10 ga and 2 16 ga wires. If one of the smaller wires is broken off or loose you can get many codes pop up.
 

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Discussion Starter #5
Update - cleaned the tightened the following grounds - 3 on the rear pass intake manifold, 2 battery terminals on intake bolts and two ground straps on the lower passenger frame.

Moved the PMD temporaryoff the extension and to the intake and have less codes - codes are now 17,18, 35,36. No other codes. Truck fires OK - slight prolonged crank and one puff of white smoke. Instantly "diesels" (heavy clanking) for 3-5 sec then a normal idle and instant check engine light.

Forgot to mention the IP filter has been removed.

Where to next?
 

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Download that pdf and start following the diagnostic tree data... start with 35 and 36... then 17 & 18. :thumb:
 

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Discussion Starter #7
17 and 18 are doable but 35 and 36 I don't have a scan tool. Is there another option if i don't have a scan tool?
 

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Mine went on/off limp mode a couple of times. The culprit was this thing:
You don't need to remove the intake, just yank it out and connect the male and female connectors back, they got the same connections as the filter.



 

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Discussion Starter #9 (Edited)
Thank you for the suggestion. I did remove that awhile back. I believe that's the IP filter. Unfortunately, it has not made a difference.

I started diagnostic testing on code 17 and of course my multi-meter is shot. Will get a new one tomorrow but the first part of the test it all passed. Now just need to backprobe the PCM to see if I'm getting 2.5 volts there. Right now the options are bad pcm or bad opti sensor.... To be continued.
 

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Gmtd scantech. It's free for diagnostics. $$$ for setting tdco.

Engh Motors - GMTDScan Basic


You'll need to buy an aldl / obd1 cable. Either serial or USB. You can get em anywhere. Ebay... and various other places. I got my cable from obd1.com I think.

May be someone nearby that'll help ya out. You can check the regional boards a little farther down the primary 'forum's page.

At some point in time you'll need it if you ever decide to mess about with your timing chain, injection pump timing etc... thankfully the setup is cheap. Better than a 5k snapon!

35/36 diagnostics are identical. You're testing the pmd itself. I dont remember if you need the scanner hooked up or not though. But its advisable to be able to hook up to the pcm and see what's going on. ESPECIALLY for 94+ ds4/electrical models.

:thumb:
 

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Discussion Starter #11
Ok - working on getting a cord. In the meantime I ran through the DTC 17 and the flow chart says bad PCM - I had reference power at PCM and 2.5 volts at cranking. Ran through the DTC 18 and the last step asks for HZ reading on the PCM which should be 60HZ. I had .005 hZ at idle and .012 at 900 rpms. Flow chart says bad opitcal sensor , harness or plug wiring.
Any ideas?
 

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If I were you, I will do the lift pump test.
Good pressure is a necessity in the running of the IP.
Also, on a 95, the OPS is the weak link since OPS controls the LP when engine is running.

You seem to skip that task, it is easy to do.
 

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... yea I did mention the lp, ops and the 94 95 ops mod.

BUT... if it dosent stall when bleeding air; 99% of the time the lp is fine and the ops obviously works. Just throwing that in there. So I cant fault if he bled some bubbles and it kept choochin.
 

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Discussion Starter #14 (Edited)
You are absolutely correct - I did not test lift pump. I just performed it however and with the engine running a bluish stream of diesel with no blips or stumbling.

I thought of something also - the truck sat for a period of time. When I first bought the truck I ran diesel conditioner and it ran awesome for a while. Then back to limp mode. I filled with fresh diesel a few months back and now getting around to working on it again. I just put in amsoil diesel conditioner - maybe the Optical sensor is just dirty. Fuel has a blueish tint too.
 

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Discussion Starter #15
Also - I've been driving the truck around just to get the diesel conditioner to work its way through the system. I notice anything above 3K rpm there is a random misfire or stumble. However, no stumble off idle up to 3K rpm.
 

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Discussion Starter #16
I have been driving the truck a bunch - not much has changed. Any ideas? I'm thinking of swapping in a Optical sensor - I know the risks of doing the swap but i just feel like that could be my issue.... Starts, runs and drives, shifts fine minus the check engine light and no power....
 

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I have been driving the truck a bunch - not much has changed. Any ideas? I'm thinking of swapping in a Optical sensor - I know the risks of doing the swap but i just feel like that could be my issue.... Starts, runs and drives, shifts fine minus the check engine light and no power....
Have you gone through the Engine harness, Optical Sensor harness, and connector pins on both?

If not, I would start there looking for worn wire insulation, frayed wires, corrosion, bent or misaligned pins.
You could have a broken wire within the insulation not showing also...

Double and triple check all grounds, that they are clean and tight.

Once you have that done then I would work on replacing the OS next..
 

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Discussion Starter #18
I think before I buy the optical sensor I will remove the upper intake and start checking the wires and will re-check grounds
 

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3000 rpms is pretty high... (I rarely get past 2k... sometimes if I'm feeling froggy 2500).

Standard 21.5:1 compression ratio. Assuming TTY head bolts (based on signature)...

The incorrect size alternator pulley will throw the tach off incrementally as you rev the engine. By about 500ish. The correct diameter is 62mm. The typical and common diameter is 57mm (what you'd get on a sbc)
 

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Discussion Starter #20
Well, still stuck after a new optical sensor change. NO CHANGE AT ALL. Starts ok after 5-10 sec of cranking and instantly get check engine light with codes 17,18, 35,36... Any new suggestions? I have a OBD1 cable and working on getting a laptop to put the software on. Are there other grounds that I'm not aware of? 3 on the back of the passenger side head, 2 battery grounds and one under the exhaust manifold on the passenger side? Could the PCM be bad? Any help is greatly appreciated...
 
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