1993 k3500 7.4 stock setup has erratic idle, cold starts runs terrible, clears up som at operating temp.

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Sev85

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Hey guys I'm at my wits end here...

I've got a 1993 k3500extended cab dually 4x4 with the 7.4 and 4l80e.

When I first bought the truck it had been sitting for a long time and the fuel pump was bad, so I just automatically replaced the tank fuel pump and fuel filter and flushed all the lines.

That was barely 6 months ago. Well fast forward that 6 months trucks been running great I had to replace the egr as it was deleted and blocked off, new o rings on the injectors, new distributor as the pick up coil died, new coil, new plugs plug wires and had to repair the wire from the knock sensor as the wire got chewed up by mice and was grounding out on the frame.

Well about a month ago it suddenly started developing a funny sputter that progressively got worse and now it's acting really wild...

So on first cold start it sputters and flutters and hunts for idle horribly, can't drop it into gear as it sputters and flutters so bad it can't pull itself under its own weight.

Once the truck warms up it clears up some and does run better bit still sounds like it has a hard miss and when half throttle or high the truck flutters hard at 2500 rpms falls on its face clears up and takes off...

When exiting a highway ramp letting off the throttle and braking after slowing down or coming to a complete stop the motor changes sound dramatically Ike its sucking air or the chokes closed and runs terrible. Pop the throttle hard something changes and the entire sound of the motor changes and it runs better.

It seems like it's running lean and I for the life cannot figure out why.

I ohm temp sensor in the intake and at 86 degree water temp it shows a ohm reading of about 106 108. Basically when it was 86 degrees it was supposed to ohm in the 2388 range and it ohm at 1138.

I checked the ohm at actual operating temperature which gauge was showing about 215 water temp was showing about 190 degrees and the ohm was coming out about 210 degrees.

Not sure if 20 degree difference could change the afr that dramatically or not which is my first question.

I checked the ohm on the iac and got 50 across all pins.

But at first start up ypu only hear the iac sucking air in when your giving it throttle and once at operating temp it stops.

I ohm the egr silenoid under vacuum and it checked out at spec and also checked voltage and everything checked at spec.

So I went to the tps, volt at closed throttle is .55 closed butterflies and 4.5 at wot.

Not perfect spec but very close.

Than moved over to map sensor which has been replaced as it was throwing a code and I checked voltages everything checked spec perfectly and ohm ground from purple wire to neg batt and resistance was like .02 on multimeter set at 2k ohm.

Put mityvac on it and checked voltages and ohm everything checked at spec.

Moved on to brake clean test had a severe air Leak on untake pulled and replaced all gaskets with felpro on intake and felpro on throttle body cleaned egr and egr ports and reinstalled.

Still same issues.

Moved on to replacing all vacuum lines on Map sensor, egr control silenoid, and tbi to evap cannister and including small rubber line from cannister to hard line that goes to tank.

Still no change...

Went on to checking wiring... found a damaged wire someone had cut and twisted a replacement wire on the red wire of pcm and electrical taped. Removed soldered in similar gauge wire and heat shrieked checked continuity on wire repair and voltage as well as resistance checked out at spec started truck still same hunting for idle and same drive ability issues..

A week or two after replacing intake gaskets and this problem starting I've now noticed when you start the truck and it builds oil pressure oil gauves maxes out passed 60 and damn near wraps the dash once it starts warm up it drops to 60 and bounces around.

So I volt checked wiring on alternator... at alternator volts are 13.96, checked terminals at battery voltage is 13.34. Should be well within requirements so I feel alternator is good.

I volt checked ground in thermostat housing mounts ground cones back great connection.

Went back to fuel pump saw ground was corroded removed cleaned frame and connector recounted and again no change.

Timing is set at 4 degrees btdc with computer unplugged and after plugged back in timing at operating temp is about 18 degrees btdc.

Checked injectors both are consistent cone shaped sprays no major dribbling and they become heavy cones when throttle is pushed.

Back story I know these trucks have issues so I installed a junction block on passenger firewall with a 6 or 8 gauge wire from negative battery terminal and ran secondary grounds to frame, to motor. To firewall.

When she's running down the highway or sitting idle the truck just seems to run lean and always runs about 215 to 230 degrees both on a mechanical gauge and on dash gauge both temp sensors are in heads.

Now when I unplug the map sensor the cel comes on and truck does not surge and hunt for idle and it runs fantastic.

I've tried multiple different iac sensors both junkyard pulls and box store replacements nothing changes except it sometimes make it run leaner and hotter.

I've tried box store tps sensors and no change. Egr is a box store replacement as I cannot find a ax Delco oem replacement anywhere but if I block off the vacuum port to egr silenoid controller there is no drive ability change.

I have tried to test fuel pressure but I can't find a single fuel pressure kit with the adapter for the tbi or the fuel filter junction I can buy to hook up a fuel gauge. But again fuel pump might barely have 1000 miles on it if that.

***edit truck does have a exhaust leak where manifolds meet y pipe where the stud pulled out and took the threads with it, but I replaced 02 sensor withing the past couple weeks and is showing no codes. I would imagine if the exhaust leak would make it run rich not lean....

I'm sorry for the ridiculous post but I'm at a complete loss here I swear I've tested and tried everything.

I Hate to give up on this truck but it's my only work truck now and I'm a full time single dad and self employed contractor so she's all I got, not to mention I literally just built a raybestos stage 3 red Kevlar clutch 4l80e with transgo towing shift inprover kit and all new **** silenoids with double clutch HD torque converter...

Please help....
 

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Schurkey

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It's possible to check all those sensors with a multimeter.

Life is too short to do that. AND, checking them with a multimeter doesn't test the wire harness they connect to. Doesn't do any good to have a functioning sensor if the signal doesn't get back to the ECM.

Connect a scan tool, look at the data stream. You can verify EVERY sensor in the time it takes to check one or two with a multimeter; and you've also verified the harnesses they connect to at the same time.

How old are the usual tune-up items? Cap, rotor, spark plugs? When you replaced the plug wires, did you put them back in the OEM looms, or are they just draped whereever they happened to fall? #5 and #7 are particularly intolerant of being routed close together.

You'll want to verify fuel pressure. Yeah, you need an adapter to do this on TBI systems. If the adapter for use at the throttle body won't fit, there's another kind that goes in place of the fuel filter. I've worked on vans where the throttle-body style won't fit.
 
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