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'94 Chevy P30 7.4 gas lost power
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twelvewheels
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Nov 27, 2013, 2:14 PM
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'94 Chevy P30 7.4 gas lost power
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1994 Chevy P30 (motorhome) 7.4 gas, 104K, I've had it since 44K Coming back from a trip it began loosing power, backfiring thru throttle body. Stopped for the night, repl. fuel filter though old one was clear. Talked to Chevy mechanic who suggested checking injector connections. Did so, bending contacts tighter. Next morning all seemed OK until faced with LONG climb. Started backfiring again, losing power. Ex. manifold got so hot it burned 5 ignition wires and ultimately got towed home. At home, first thing, repl. wires. Checked fuel press: 32psi, which is spec. for this year. Checked exhaust back pressure on road test: no more than 3psi at full throttle. Power still way down but no more backfiring. Checked timing: 14 deg. retarded w. timing connector DISCONNECTED. Should be 4 deg. adv! 0 deg w. connector connected. Scan tool show 0 deg. knock sensor retard. Disconnected knock sensor: no change in timing. Took ICM to Autozone to be tested: checked OK. I can't imagine the timing chain jumped: ign. timing doesn't jump around plus engine has had regular oil changes. Bad PCM maybe? Maybe Autozone tester doesn't check timing portion of ICM? Can anyone please help? I'm stumped. Tom
(This post was edited by twelvewheels on Nov 27, 2013, 2:30 PM)
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Tom Greenleaf
Ultimate Carjunky
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Nov 27, 2013, 2:59 PM
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Re: '94 Chevy P30 7.4 gas lost power
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It would be a crime if they used a nylon cam gear on such a critter and HD engine but you wouldn't know till looking at it. It shouldn't be worn out IMO but could have suffered plastic failure??
If and only if that's still on the maybe list you can watch distributor rotor while turning crank bolt back and forth hopefully either damper or other has a marker showing degrees.
Have it tight one way (all this engine plain off and cold) unseen is tight on one side then see how many degrees crank turns before rotor turns at distributor. 5 is the scare line IMO and by 7 I'd be concerned if you can get that accurate.
Totally general on just that, again if that at all, is they run fine then can do the backfire thing, loss of power and sometimes magically jump back an behave OR engine quits running, compression way too low and without testing cranks like plugs are out if off enough.
Other is a slight maybe is that distributor gear is shearing its roll pin if that's how that's done or shaft bearing so worn it can mess up the show. When shaft bearings are way too worn I think I recall raising idle speed while watching the dwell would drop excessively or would result in more advance than designed for just for that test.
Hard to conclude some of this and harder as IDK what specs could be changed for an engine destined to be for a motor home sold as a chassis no doubt expecting what will be done to it I think. Sorry to be vague on just what and how these things get made new but would think the coach maker would order what was intended.
Sorry can't be more specific so take that as just some ideas and things I've personally run into with assorted engines of the era and older too. Not my game as these things never fit anywhere I ever worked - looked a several on location for assorted things is about it,
T
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twelvewheels
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Nov 27, 2013, 3:10 PM
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Re: '94 Chevy P30 7.4 gas lost power
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Thanks for your help Tom! My research shows they used steel gears on this truck engine. I wish it were easy to turn the crank on this beast but front bolt is buried in the fan shroud. Maybe I should retrofit to points ignition  Tom
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Tom Greenleaf
Ultimate Carjunky
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Nov 28, 2013, 1:36 AM
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Re: '94 Chevy P30 7.4 gas lost power
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Your info will plain be better than what I can search out hoping you have some info on what the powertrain really is and ordered to be. Forget points as I don't think the even an entire distributor will fit or work in this at all.
I think you would be correct that timing gear is metal not those plastic glued on outer cam gear thing but still should be ruled out.
Your description of loss of power and backfire is "a" a key clue or "b" THE clue. That totally suggests early ignition. I'm not sure you just turn distributor in this critter for setting exact timing - you may know better on that. Bet it sets in one position and is PCM controlled as you mentioned knock sensor and things all suggesting a 90s generation fuel and ignition set up. Those would require more than physical setting and may lack timing marks completely with real #s.
Would love to rule out the chain anyway and you say crank bolt is well hidden. All metal or not RV things including marine (that I did a lot of) quite frequently have long down times or stored for off seasons which creates issues alone. Fuels don't like time and the ethanol is a problem on top.
See what you can do to check that chain. Breaker bar and socket on that crank bolt or maybe you can turn "flywheel" ring gear. This just some info gathering all harder on an engine that isn't even running (is it?) enough for other tests.
Compression testing of just some easy ones could help too.
If this has (probably does) have an OBDI port get the codes. It doesn't if you don't have a "check engine" light that should light as a bulb test with key just in run position not trying to start it at all. Some, if bulb doesn't work and does have the set up causes problems.
Lots to think about. Happy Thanksgiving. Hope you have this at a destination you can deal with as at least around me shops, techs, parts seems to end for a few days around now and could be a mad rush Monday even if you did have this pin pointed,
Tom
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twelvewheels
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Nov 30, 2013, 1:48 PM
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Re: '94 Chevy P30 7.4 gas lost power
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Hi Tom, I just got around to measuring the slop in my timing chain: 3 deg.. I clamped a long pointer to the distr. shaft so I could see it from below and I preloaded it with a rubber band to take out the slop in the distr. gears.. BTW, the gears have about 2 deg. slop at the distr. Soooo I suppose I'll pull the distr. to see if anything is amiss, unless you have any other ideas??? Tom
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Tom Greenleaf
Ultimate Carjunky
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Nov 30, 2013, 4:41 PM
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Re: '94 Chevy P30 7.4 gas lost power
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The degrees of slop/lash I meant was how far crank damper can turn in degrees. I plead I just don't know what exact setting or view you have and how you figured degrees. WARNING on removing the distributor as it might take setting base timing incredibly close then is not adjustable! That type will require setting it an entirely different way than a timing light and look for marks while turning distributor - wait for the guys to chime in I hope on this engine for this year for that.
I think your chain is probably fine and timing issue is NOT position of distributor unless somehow it moved by itself? NOT likely.
My source for procedures really lacked - plain had to be wrong IMO, it happens.
Please wait for some ideas from the guys. I don't want to make this more of a problem to diagnose than it is for you now or cause another tow to a place that can deal with this. Arggh and sorry but right now I'm out of any ideas dangit!
T
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twelvewheels
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Nov 30, 2013, 4:57 PM
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Re: '94 Chevy P30 7.4 gas lost power
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Actually Tom, I'm pretty sure I found the problem: I pulled the distributor and found the magnetic star wheel to be quite loose on the shaft. The area was kind of rusty so after cleaning and examining with a magnifier I found that the star wheel hub was actually cracked. I suspect the expanding rust cracked it. A new distr. is only $125 from Autozone and less than $60 on ebay. BTW I measured chain play at the crankshaft timing tab.......3 deg. Thanks for the support, Tom
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Hammer Time
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Nov 30, 2013, 5:01 PM
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Re: '94 Chevy P30 7.4 gas lost power
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Yes, the timing is adjustable on this one however I don't think that is the origin of your problems. Starting to backfire at a steady throttle position is usually a sign of the mixture going lean for one reason or another and that's what I would concentrate on. I would rig up a fuel pressure gauge and a scan tool for a road test and observe what happens to the fuel pressure and the adaptives when the problem occurs.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Tom Greenleaf
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Dec 1, 2013, 1:41 AM
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Re: '94 Chevy P30 7.4 gas lost power
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Thanks Hammer. Obviously was soooo unsure of this engine set up.
OP = Original poster: You found it, great. 3 degrees is nothing and a new chain would have that much in no time but stay there for ages the way I would test one. Not so common on the GM workhorse blocks, 350, 454 = 5.7, 7.4 respectively. Those two are very popular for assorted non standard work horse use both road RV and Marine which I know a bit better to watch out for custom changes and no info easy to find for me anyway.
The marine stuff had to change things a lot if nothing but spark arresting things on everything so a yacht didn't just blow up - not the hit of the party at a marina!
Seems you are all set. Glad for that,
Tom
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