Please help, the auto parts stores are out of dynamite and I've been working on this truck for over a week straight.

The whole thing started with the surging hesitation once the engine got to temp. I could go maybe 10-15 miles and it gradually became less and less until I couldn't go 3 miles before the chugging started. Started looking at this forum for info and checked all kinds of stuff. Replaced some sensors because they were pretty hammered or VERY dirty. Once I shorted the #6 pin on the self test connector to ground, I can get the fuel pump relay to run and get fuel to at least the fuel filter and the hi pressure pump is humming at this point. But truck will still not start. With the ground wire on #6 removed, I get nothing but engine will crank and will stumble if I shoot some ether in it, so I've got spark. I just replaced the EEC relay but it still won't energize the FPR on it's own. Is the ECM bad?

Stuff replaced so far due to error codes or just a shot in the dark: fuel pump relay, EEC relay, air charge temp sensor, engine temp sensor (with an aftermarket guage), TPS, EGR position sensor, ignition switch, fuel filter on the frame rail, ignition coil, cap rotor and wires.

As far as I can tell, I do not have a fuel pressure check point, so I can't verify the fuel pressure to the rail.

I'm going to check wire connections to the ECM since that seems to be all that's left. According to the diagram I have, the yellow is hot at all times and the red wires on the fuel injectors, EEC relay and FPR get their power from the ECM.

I am pretty much at my wits end and any info is much appreciated.

Joel
norcal

 

Pull the ECM and smell it. If it smells burnt, you just found your problem. You can also pull the cover off it and look for damage.

 

Thanks for the reply.

Pulled the ECM and saw no signs of smoked parts and no burnt electrical smell. About the only thing I could see that might be bad was one electrolytic capacitor that looked like it might be starting to leak. Nothing else bad visually.

Cut the connector to the fuel pump relay and put regular female spade connectors on it. No joy. Everything powers up if you ground #6 terminal on the self test connector, but it still won't start with even that grounded.

No power to the injectors maybe?

I will look for anymore fusible links and double check all fuses again.

Joel

 

The leaky cap might be causing it. Reman ECM's are only about 100 bucks. Just saying.

 

Most of the fusible links are at the starter relay, but there's at least one near the FP & EEC relays, and probably another buried in the dash harness. Click this & read the caption:

 

Checked all of the fusible links and all were good. Proceeded to check the entire wiring harness and found several spots that had bad or no insulation at all so, so got those fixed up. With as many bad spots as I had on the harness, I'm surprised that it ran.

I had replaced the EGR position sensor because it was recommended by someone else as being a possible problem and on a whim I pulled that EGR sensor bypass assy off and put one on that I got from a junkyard. No joy, so I pulled the EGR bypass assy off the intake manifold and with the help of my brother, it started right up. One of the valve seemed to move more than the other and I read somewhere that a penny on top of the diaphram will sometimes make up for the difference in diaphrams and sensor plungers length. Tried the penny and it works.

At least until I get to 35mph and then it cuts out until the speed gets down to 25mph and then it runs again until the process repeats. I can hear a hissing sound coming from the back of the engine near that crossover selenoid. No leaks though. There is also some checkvalves back there too, but are hard to get to. If one of those fails, what will the symptoms be?

I may have found the fuel test valve but it isn't where anyone says it is. It looks like it might be back where the fuel line attaches to the rails on driver side cylinder bank. I've got these square fuel rails, so maybe that has something to do with it.

Any help from you guys is GREATLY appreciated. You have no idea.

Joel
norcal

 

That hissing sound is rhythmic, like it's coming from the smog pump. As you are cranking the engine, it's about one hiss every half second or so.

 

Checked the cap and rotor, and they were a bit corroded, so I replaced those. Thought they were new, but I guess not. Drove up the road 1/2 mile and same symptoms. Chugging/surging off of idle and won't go past 35 mph. Still cannot find fuel test point. I will check the FPR to see if fuel comes out the top. That might explain the occasional fuel smell I get after shutdown. Thanks

 

Didn't see any fuel coming out of the FPR and disconnecting the vacuum line and capping it off did not change anything. Same symptoms.

 

Forgot to mention that the codes pulled are 63, 10, 31
But even after letting the truck sit for 24 hours with the pos lead disconnected, I still get the same symptoms. So that should have reset the computer.

 

According to my books, the 31 code says the EGR is out of range or it is not in its normal closed position.
Code 63 says the throttle position sensor is giving low voltage signal to the PCM/EEC.
I find no 10 code in the book or database.

Swapping out the complete EGR MIGHT cure both issues. The TPS will read low if there is air coming into the intake that does not pass thru the throttle body. I'd also check the power brake hose to the booster for leaks. Frankly, I'd cut off all vacuum to everything until you find the issue as you can have multiple issues which will have you in circles.

 

Just found a thread on rough idle on an 86 and it looks like my idle screw may be set improperly. I pulled the IAC and it made no difference whatsoever. So now I get to chase that...

 

Okay, found out what the 10 code means. It's a separator to let you know the codes are repeating.

What I decided to do was take everything back to original, at least then I could drive it a bit further before it started to act up. So right now we are back to all original sensors except the TPS, the distributor TFI module, and the cap and rotor. And the temp gauge sensor, water pump and thermostat and fuel canister filter.

Still get the 63 and 31 codes. And the surging chugging only started happening once the engine temp got to 200f. until then, it ran GREAT. Granted this was only 6 miles and several stops, but it ran great until I hooked up the flatbed with a load of water on it.

When it was surging, the fuel pump relay was chattering and so was fuel pressure regulator up on the fuel rail. Not sure if this rig even has a fuel test point and the rails are square, not round. But the FPR was definitely cycling off and on.

 

No, that's not what the separator pulse means. It separates the test codes from the continuous-memory codes. You'd be better-off reading codes with a jumper wire (paperclip) than a scanner.

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How are you measuring engine temperature? (...and how do you load water on a flatbed?)

Did you change the FP relay to the newer style? The FPR can't "cycle on & off", and you can't hear what it's doing. It's just a diaphragm-operated valve.



This diagram shows your early fuel rails without test port. The one in the lower R corner (and the 2nd thumbnail) is the later, common style, and you can see the test port on it.

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The hiss behind the engine is probably the most-significant symptom you've mentioned. Find it.



It's likely the plenum-to-lower-intake gasket (9H486 in that diagram above), which is known to suck in, creating a massive vacuum leak. That could have caused vacuum pulsation inside the FPR, as well as many of your other symptoms.