Runs lean and fails an emissions test
One truck in the cited thread ran lean and failed its emissions test on both oxides of nitrogen and unburnt hydrocarbons, and members pointed at a clogged exhaust gas recirculation path and at vacuum leaks. This is a single thread and it is tiered accordingly, but the shape of the advice is worth having because it says something specific about where to start: on a forty-year-old truck the rubber is the suspect before the metering is. Emissions rules are not universal — what a truck must pass, and whether it must be tested at all, depends entirely on the jurisdiction, and a US state test, a Costa Rican inspection and a British test measure different things to different limits. Treat the numbers on your own test slip as the evidence and this page as a method. Nothing here changes how the truck should be driven; it changes what it costs you to run and whether it passes wherever you live.
Gen 11983–1991USWorks worseAnecdotalSafety-critical
Drive normallyManeje normalmente
filteredDoes not fit the vehicle you selected
Provisional matchThis is safety-critical, and the match to your truck is only provisional: it was made on generation, market, year and engine alone. Narrow your selection, and confirm against your own vehicle, before acting on anything here.
Safety notice — Fuel system.
This affects a safety-critical system. Reference material only: for safety-critical work, consult a qualified mechanic.
Esto afecta un sistema crítico para la seguridad. Material de referencia únicamente: en trabajos críticos para la seguridad, consulte a un mecánico calificado.
Anecdotal. This entry is not backed by factory documentation — treat its values and steps as a starting point, not as if they came from the factory manual.
Anecdótico. Esta entrada no se apoya en documentación de fábrica — tome los valores y los pasos como punto de partida, no como si vinieran del manual de fábrica.
Symptoms
- The truck fails its emissions test, typically on unburnt hydrocarbons or oxides of nitrogen
- It hesitates, stumbles or flat-spots as you pull away
- The idle wanders up and down, or sits higher than it should
- A light metallic pinging under load on a hill
Likely causes
- A vacuum leak is letting unmetered air into the engine
- The exhaust gas recirculation path is clogged with carbon and no longer doing its job
- Decades-old rubber hoses have gone hard and are leaking at several points at once
- The fuel metering itself is worn or out of adjustment
Diagnostic steps
- Read the actual numbers on the test slip and note which pollutants failed and by how much, rather than working from the pass or fail alonePoints toA vacuum leak is letting unmetered air into the engine · The exhaust gas recirculation path is clogged with carbon and no longer doing its job
- With the engine idling, work systematically along every vacuum hose and joint listening and watching for a change in idle speedPoints toA vacuum leak is letting unmetered air into the engine · Decades-old rubber hoses have gone hard and are leaking at several points at once
- Take the EGR valve off and look at the valve and its passage for carbon packed hard enough to block itPoints toThe exhaust gas recirculation path is clogged with carbon and no longer doing its job
- Only after every leak is cured, check the fuelling and adjustment — a leak makes every fuelling measurement liePoints toThe fuel metering itself is worn or out of adjustmentRules outA vacuum leak is letting unmetered air into the engine
Fix paths
Renew all the vacuum hoses as a set
$Difficulty 2/5
On a truck this age, chasing one leak at a time is slower and dearer than replacing the lot. Hose is cheap; the diagnosis time is not.
Clean or replace the EGR valve and its passage
$–$$Difficulty 3/5
Clean the valve and the passage behind it. A blocked passage with a perfectly good valve bolted to it is a common and frustrating way to spend money on the wrong part.
Overhaul or re-adjust the fuel metering
$$–$$$Difficulty 4/5
Last, not first. Every measurement you take before the leaks are fixed will send you here for the wrong reason.